Editing Dark matter (section)

== Composition ==
[[File:Dark matter candidates.pdf|thumb|upright=1.9|Different dark matter candidates as a function of their mass in units of electronvolt (eV)]]

The identity of dark matter is unknown, but there are many [[hypothesis|hypotheses]] about what dark matter could consist of, as set out in the table below.
{| class = wikitable
|+ Some dark matter hypotheses
|-
|rowspan=3| [[light boson]]s
| [[quantum chromodynamics]] [[axion]]s
|-
| [[WISP (particle physics)|axion-like particle]]s
|-
| [[fuzzy cold dark matter]]
|-
|rowspan=2| [[neutrino]]s
| [[neutrino|Standard Model]]{{efn|The three neutrino types already observed are indeed abundant, and dark, and matter, but their individual masses are almost certainly too tiny to account for more than a small fraction of dark matter, due to limits derived from [[observable universe|large-scale structure]] and high-[[redshift]] galaxies.<ref name="bertone merritt" />}}
|-
| [[sterile neutrinos]]
|-
|rowspan=4| other particles
| [[lightest supersymmetric particle]]
|-
| [[weakly interacting massive particle]]
|-
| [[self-interacting dark matter]]
|-
| [[atomic dark matter]]<ref>{{cite journal |last1=Bansal |first1=Saurabh |last2=Barron |first2=Jared |last3=Curtin |first3=David |last4=Tsai |first4=Yuhsin |date=2023-10-16 |title=Precision cosmological constraints on atomic dark matter |url=https://doi.org/10.1007/JHEP10(2023)095 |journal=Journal of High Energy Physics |language=en |volume=2023 |issue=10 |pages=95 |doi=10.1007/JHEP10(2023)095 |arxiv=2212.02487 |bibcode=2023JHEP...10..095B |issn=1029-8479}}</ref><ref>{{citation |last1=Bansal |first1=Saurabh |title=Precision Cosmological Constraints on Atomic Dark Matter |date=2023-07-27 |arxiv=2212.02487 |last2=Barron |first2=Jared |last3=Curtin |first3=David |last4=Tsai |first4=Yuhsin |journal=Journal of High Energy Physics |volume=2023 |issue=10 |page=95 |doi=10.1007/JHEP10(2023)095 |bibcode=2023JHEP...10..095B |quote=leading to a better fit than ΛCDM or ΛCDM + dark radiation}}</ref><ref>{{cite web |last=Sutter |first=Paul Sutter |date=2023-06-07 |title=Dark matter atoms may form shadowy galaxies with rapid star formation |url=https://www.space.com/dark-matter-atoms-form-stars-galaxies-simulations |access-date=2024-01-09 |website=Space.com |language=en}}</ref><ref name="MirrorStars">{{cite journal |last1=Armstrong |first1=Isabella |display-authors=etal |date=2024 |title=Electromagnetic Signatures of Mirror Stars |journal=The Astrophysical Journal |volume=965 |issue=1 |page=42 |arxiv=2311.18086 |bibcode=2024ApJ...965...42A |doi=10.3847/1538-4357/ad283c |doi-access=free}}</ref>
|-
| [[strangelet]]<ref>{{cite journal |last1=VanDevender |first1=J. Pace |last2=VanDevender |first2=Aaron P. |last3=Sloan |first3=T. |last4=Swaim |first4=Criss |last5=Wilson |first5=Peter |last6=Schmitt |first6=Robert G. |last7=Zakirov |first7=Rinat |last8=Blum |first8=Josh |last9=Cross |first9=James L. |last10=McGinley |first10=Niall |date=2017-08-18 |title=Detection of magnetized quark-nuggets, a candidate for dark matter |journal=Scientific Reports |language=en |volume=7 |issue=1 |page=8758 |doi=10.1038/s41598-017-09087-3 |pmid=28821866 |pmc=5562705 |arxiv=1708.07490 |bibcode=2017NatSR...7.8758V |issn=2045-2322}}</ref>
|-
| [[dynamical dark matter]]<ref name=DienesThomas2012>{{cite journal |last1=Dienes |first1=Keith R. |last2=Thomas |first2=Brooks |date=2012-04-24 |title=Dynamical dark matter. I. Theoretical overview |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.85.083523 |journal=Physical Review D |volume=85 |issue=8 |pages=083523 |doi=10.1103/PhysRevD.85.083523|arxiv=1106.4546 |bibcode=2012PhRvD..85h3523D }}</ref>
|
|-
|rowspan=3| [[Macroscopic scale|macroscopic]]
| [[primordial black hole]]s<ref name="Carr24"/><ref name="Bird"/><ref name="jwst">{{cite journal |last1=Hütsi |first1=Gert |last2=Raidal |first2=Martti |last3=Urrutia |first3=Juan |last4=Vaskonen |first4=Ville |last5=Veermäe |first5=Hardi |date=2 February 2023 |title=Did JWST observe imprints of axion miniclusters or primordial black holes? |journal=Physical Review D |volume=107 |issue=4 |page=043502 |arxiv=2211.02651 |bibcode=2023PhRvD.107d3502H |doi=10.1103/PhysRevD.107.043502 |s2cid=253370365}}</ref><ref name="Carr">{{cite journal |last1=Carr |first1=Bernard |last2=Kühnel |first2=Florian |title=Primordial black holes as dark matter candidates |journal=SciPost Physics Lecture Notes |date=2 May 2022 |page=48 |doi=10.21468/SciPostPhysLectNotes.48 |s2cid=238407875 |url=https://scipost.org/SciPostPhysLectNotes.48/pdf |access-date=13 February 2023 |doi-access=free |arxiv=2110.02821 }} (See also the [https://indico.cern.ch/event/949654/contributions/4031007/attachments/2293539/3901659/Carr-Kuhnel.pdf accompanying slide presentation.]</ref><ref name="Espinosa">{{cite journal |last1=Espinosa |first1=J. R. |last2=Racco |first2=D. |last3=Riotto |first3=A. |title=A Cosmological Signature of the Standard Model Higgs Vacuum Instability: Primordial Black Holes as Dark Matter |journal=Physical Review Letters |volume=120 |issue=12 |page=121301 |doi=10.1103/PhysRevLett.120.121301 |pmid=29694085 |date=23 March 2018|arxiv=1710.11196 |bibcode=2018PhRvL.120l1301E |s2cid=206309027 }}</ref><ref name="Clesse">{{cite journal |last1=Clesse |first1=Sebastien |last2=García-Bellido |first2=Juan |title=Seven Hints for Primordial Black Hole Dark Matter |journal=Physics of the Dark Universe |volume=22 |pages=137–146 |arxiv=1711.10458 |bibcode=2018PDU....22..137C |doi=10.1016/j.dark.2018.08.004 |year=2018 |s2cid=54594536 }}</ref><ref name="Lacki">{{cite journal |last1=Lacki |first1=Brian C. |last2=Beacom |first2=John F. |title=Primordial Black Holes as Dark Matter: Almost All or Almost Nothing |journal=The Astrophysical Journal |date=12 August 2010 |volume=720 |issue=1 |pages=L67–L71 |doi=10.1088/2041-8205/720/1/L67 |language=en |issn=2041-8205 |arxiv=1003.3466 |bibcode=2010ApJ...720L..67L |s2cid=118418220 }}</ref><ref name="Kashlinsky">{{cite journal |last1=Kashlinsky |first1=A. |title=LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies |journal=The Astrophysical Journal |date=23 May 2016 |volume=823 |issue=2 |pages=L25 |doi=10.3847/2041-8205/823/2/L25 |issn=2041-8213|arxiv=1605.04023 |bibcode=2016ApJ...823L..25K |s2cid=118491150 |doi-access=free }}</ref><ref name="Frampton">{{cite journal |last1=Frampton |first1=Paul H. |last2=Kawasaki |first2=Masahiro |last3=Takahashi |first3=Fuminobu |last4=Yanagida |first4=Tsutomu T. |title=Primordial Black Holes as All Dark Matter |journal=Journal of Cosmology and Astroparticle Physics |date=22 April 2010 |volume=2010 |issue=4 |page=023 |doi=10.1088/1475-7516/2010/04/023 |issn=1475-7516|arxiv=1001.2308 |bibcode=2010JCAP...04..023F |s2cid=119256778 }}</ref><ref name="Carneiro">{{cite journal |last1=Carneiro |first1=S. |last2=de Holanda |first2=P.C. |last3=Saa |first3=A. |title=Neutrino primordial Planckian black holes |journal=Physics Letters |date=2021 |volume=B822 |page=136670 |doi=10.1016/j.physletb.2021.136670 |issn=0370-2693|bibcode=2021PhLB..82236670C |s2cid=244196281 |doi-access=free |hdl=20.500.12733/1987 |hdl-access=free }}</ref>
|-
| [[massive compact halo objects]] (MACHOs)
|-
| [[macroscopic dark matter]] (Macros)
|}

[[File:Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter.ogv|thumb|[[Fermi Gamma-ray Space Telescope|Fermi-LAT]] observations of dwarf galaxies provide new insights on dark matter.]]

=== Baryonic matter ===
{{Distinguish|Missing baryon problem}}

Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard [[baryon|baryonic matter]], such as protons or neutrons. Most of the ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, fall into this category.<ref name=GianfracoHooperHistory>{{cite journal |last1=Bertone |first1=Gianfranco |last2=Hooper |first2=Dan |title=History of dark matter |journal=Reviews of Modern Physics |date=15 October 2018 |volume=90 |issue=4 |page=045002 |doi=10.1103/RevModPhys.90.045002|arxiv=1605.04909 |bibcode=2018RvMP...90d5002B |s2cid=18596513 }}</ref><ref name=BaryonicSource01>{{cite web |url=http://astronomy.swin.edu.au/cosmos/B/Baryonic+Matter |title=Baryonic Matter |website=COSMOS – The SAO Encyclopedia of Astronomy |publisher=[[Swinburne University of Technology]]|access-date=16 November 2022}}</ref> A black hole would ingest both baryonic and non-baryonic matter that comes close enough to its event horizon; afterwards, the distinction between the two is lost.<ref>{{cite web |title=Baryonic Matter |url=https://astronomy.swin.edu.au/cosmos/b/Baryonic+Matter |access-date=2023-10-03 |website=astronomy.swin.edu.au |publisher=Cosmos: The Swinburne Astronomy Online Encyclopedia |publication-place=Melbourne, Victoria, Australia: Swinburne University of Technology}}</ref> 

These massive objects that are hard to detect are collectively known as [[Massive compact halo object|MACHO]]s. Some scientists initially hoped that baryonic MACHOs could account for and explain all the dark matter.<ref name=Randall_2015/>{{rp|286}}<ref>{{cite news |title=MACHOs may be out of the running as a dark matter candidate |url=https://astronomy.com/news/2016/08/machos-may-be-out-of-the-running-as-a-dark-matter-candidate |access-date=16 November 2022 |work=Astronomy.com |date=2016 |language=en}}</ref>

However, multiple lines of evidence suggest the majority of dark matter is not baryonic:
* Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.
* The theory of [[Big Bang nucleosynthesis]] predicts the observed [[abundance of the chemical elements]]. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.<ref>{{cite book |author=Weiss, Achim |url=http://www.einstein-online.info/spotlights/BBN |title=Big bang nucleosynthesis: Cooking up the first light elements |archive-url=https://web.archive.org/web/20130206021217/http://www.einstein-online.info/spotlights/BBN |archive-date=6 February 2013 |publisher=Einstein Online |volume=2 |year=2006 |page=1017 |access-date=1 June 2013 |url-status=dead }}</ref><ref>{{cite book |last1=Raine |first1=D. |last2=Thomas |first2=T. |date=2001 |title=An Introduction to the Science of Cosmology |page=30 |publisher=[[IOP Publishing]] |isbn=978-0-7503-0405-4 |oclc=864166846 }}</ref> Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's [[Friedmann equations#Density parameter|critical density]]. In contrast, [[large-scale structure of the universe|large-scale structure]] and other observations indicate that the total matter density is about 30% of the critical density.<ref name="planckesa2015" />
* Astronomical searches for [[gravitational microlensing]] in the [[Milky Way]] found at most only a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30&nbsp;solar masses, which covers nearly all the plausible candidates.<ref>{{cite journal |last1=Tisserand |first1=P. |last2=Le&nbsp;Guillou |first2=L. |last3=Afonso |first3=C. |last4=Albert |first4=J.N. |last5=Andersen |first5=J. |last6=Ansari |first6=R. |last7=Aubourg |first7=É. |last8=Bareyre |first8=P. |last9=Beaulieu |first9=J.P. |last10=Charlot |first10=X. |last11=Coutures |first11=C. |last12=Ferlet |first12=R. |last13=Fouqué |first13=P. |last14=Glicenstein |first14=J.F. |last15=Goldman |first15=B. |last16=Gould |first16=A. |last17=Graff |first17=D. |last18=Gros |first18=M. |last19=Haissinski |first19=J. |last20=Hamadache |first20=C. |last21=De Kat |first21=J. |last22=Lasserre |first22=T. |last23=Lesquoy |first23=É. |last24=Loup |first24=C. |last25=Magneville |first25=C. |last26=Marquette |first26=J.B. |last27=Maurice |first27=É. |last28=Maury |first28=A. |last29=Milsztajn |first29=A. |last30=Moniez |first30=M. |display-authors=6 |title=Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds |journal=Astronomy and Astrophysics |volume=469 |issue=2 |pages=387–404 |year=2007 |doi=10.1051/0004-6361:20066017 |url=https://www.researchgate.net/publication/41714676 |arxiv=astro-ph/0607207 |bibcode=2007A&A...469..387T|s2cid=15389106 }}</ref><ref>{{cite journal |last1=Graff |first1=D. S. |last2=Freese |first2=K. |year=1996 |title=Analysis of a ''Hubble Space Telescope'' Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo |journal=The Astrophysical Journal |volume=456 |issue=1996 |page=L49 |arxiv=astro-ph/9507097 |bibcode=1996ApJ...456L..49G |doi=10.1086/309850 |s2cid=119417172}}</ref><ref>{{cite journal |last1=Najita |first1=J. R. |last2=Tiede |first2=G. P. |last3=Carr |first3=J. S. |year=2000 |title=From Stars to Superplanets: The Low-Mass Initial Mass Function in the Young Cluster IC 348 |journal=The Astrophysical Journal |volume=541 |issue=2 |pages=977–1003 |arxiv=astro-ph/0005290 |bibcode=2000ApJ...541..977N |doi=10.1086/309477 |s2cid=55757804}}</ref><ref>{{cite journal |arxiv=1106.2925 |title=The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs |journal=Monthly Notices of the Royal Astronomical Society |volume=416 |issue=4 |pages=2949–2961 |last1=Wyrzykowski |first1=L. |last2=Skowron |first2=J. |last3=Kozlowski |first3=S. |last4=Udalski |first4=A. |last5=Szymanski |first5=M.K. |last6=Kubiak |first6=M. |last7=Pietrzynski |first7=G. |last8=Soszynski |first8=I. |last9=Szewczyk |first9=O. |last10=Ulaczyk |first10=K. |last11=Poleski |first11=R. |last12=Tisserand |first12=P. |display-authors=6 |doi=10.1111/j.1365-2966.2011.19243.x |year=2011 |doi-access=free |bibcode=2011MNRAS.416.2949W|s2cid=118660865 }}</ref><ref>{{cite arXiv |title=Death of stellar baryonic dark matter candidates |first1=Katherine |last1=Freese |eprint=astro-ph/0007444 |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2000}}</ref><ref>{{cite book |pages=4–6 |first1=Katherine |last1=Freese |arxiv=astro-ph/0002058 |bibcode=2000fist.conf...18F |doi=10.1007/10719504_3 |chapter=Death of Stellar Baryonic Dark Matter |series=ESO Astrophysics Symposia |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2003 |title=The First Stars |isbn=978-3-540-67222-7 |citeseerx=10.1.1.256.6883|s2cid=119326375 }}</ref>
* Detailed analysis of the small irregularities (anisotropies) in the [[cosmic microwave background]] by [[WMAP]] and [[Planck (spacecraft)|Planck]] indicate that around five-sixths of the total matter is in a form that only interacts significantly with ordinary matter or [[photon]]s through gravitational effects.<ref>{{cite journal |first1=L. |last1=Canetti |first2=M. |last2=Drewes |first3=M. |last3=Shaposhnikov |title=Matter and Antimatter in the Universe |journal=New J. Phys. |year=2012 |volume=14 |issue=9 |page=095012 |doi=10.1088/1367-2630/14/9/095012 |arxiv=1204.4186 |bibcode=2012NJPh...14i5012C|s2cid=119233888 }}</ref>

=== Non-baryonic matter ===
There are two main candidates for non-baryonic dark matter: new particles and [[primordial black hole]]s. 

Unlike baryonic matter, nonbaryonic particles do not contribute to the formation of the [[Chemical element|elements]] in the early universe ([[Big Bang nucleosynthesis]])<ref name="Copi 1995" /> and so its presence is felt only via its gravitational effects (such as [[weak lensing]]). In addition, some dark matter candidates can interact with themselves ([[self-interacting dark matter]]) or with ordinary particles (e.g. [[Weakly Interacting Massive Particle|WIMP]]s or Weakly Interacting Massive Particles), possibly resulting in observable by-products such as [[gamma rays]] and neutrinos (indirect detection).<ref name="bertone merritt">{{cite journal |last1=Bertone |first1=G. |last2=Merritt |first2=D. |title=Dark Matter Dynamics and Indirect Detection |year=2005 |journal=[[Modern Physics Letters A]] |volume=20 |issue=14 |pages=1021–1036 |arxiv=astro-ph/0504422 |bibcode=2005MPLA...20.1021B |doi=10.1142/S0217732305017391|s2cid=119405319 }}</ref> Candidates abound (see the table above), each with their own strengths and weaknesses.

==== Undiscovered massive particles ====
{{main|Weakly Interacting Massive Particles}}
There exists no formal definition of a Weakly Interacting Massive Particle, but broadly, it is an [[elementary particle]] which interacts via [[gravity]] and any other force (or forces) which is as weak as or weaker than the [[weak nuclear force]], but also non-vanishing in strength. Many WIMP candidates are expected to have been produced thermally in the early Universe, similarly to the particles of the Standard Model<ref>{{cite journal | last = Garrett | first = Katherine | title = Dark matter: A primer | year = 2010 | journal = Advances in Astronomy | volume = 2011 | issue = 968283 | pages = 1–22 | doi = 10.1155/2011/968283| arxiv = 1006.2483 | bibcode = 2011AdAst2011E...8G | doi-access = free }}</ref> according to [[Big Bang]] cosmology, and usually will constitute [[cold dark matter]]. Obtaining the correct abundance of dark matter today via [[thermal production]] requires a self-[[annihilation]] [[Cross section (physics)|cross section]] of <math>\langle \sigma v \rangle</math> ≃ {{val|3|e=-26|u=cm<sup>3</sup>⋅s<sup>−1</sup>}}, which is roughly what is expected for a new particle in the 100&nbsp;[[GeV]]/''c''<sup>2</sup> mass range that interacts via the [[electroweak force]].

Because [[supersymmetry|supersymmetric]] extensions of the Standard Model of particle physics readily predict a new particle with these properties, this apparent coincidence is known as the "'''WIMP miracle'''", and a stable supersymmetric partner has long been a prime explanation for dark matter.<ref>{{cite journal |last1=Jungman |first1=Gerard |last2=Kamionkowski |first2=Marc |last3=Griest |first3=Kim |year=1996 |title=Supersymmetric dark matter |journal=Physics Reports |volume=267 |issue=5–6 |pages=195–373 |s2cid=119067698 |arxiv=hep-ph/9506380 |bibcode=1996PhR...267..195J |doi=10.1016/0370-1573(95)00058-5}}</ref> Experimental efforts to detect WIMPs include the search for products of WIMP annihilation, including [[gamma ray]]s, [[neutrino]]s and [[cosmic ray]]s in nearby galaxies and galaxy clusters; direct detection experiments designed to measure the collision of WIMPs with [[Atomic nucleus|nuclei]] in the laboratory, as well as attempts to directly produce WIMPs in colliders, such as the [[Large Hadron Collider]] at [[CERN]]. 

In the early 2010s, results from [[#Direct detection|direct-detection]] experiments along with the lack of evidence for supersymmetry at the [[Large Hadron Collider]] (LHC) experiment<ref>{{cite news |url=http://news.discovery.com/space/lhc-discovery-maims-supersymmetry-again-130724.htm |title=LHC discovery maims supersymmetry again |website=Discovery News}}</ref><ref>{{cite arXiv |last=Craig |first=Nathaniel |year=2013 |title=The State of Supersymmetry after Run I of the LHC |class=hep-ph  |eprint=1309.0528}}</ref> have cast doubt on the simplest WIMP hypothesis.<ref>{{cite journal |last1=Fox |first1=Patrick J. |last2=Jung |first2=Gabriel |last3=Sorensen |first3=Peter |last4=Weiner |first4=Neal |year=2014 |title=Dark matter in light of LUX |journal=Physical Review D |volume=89 |issue=10 |page=103526 |arxiv=1401.0216 |bibcode=2014PhRvD..89j3526F |doi=10.1103/PhysRevD.89.103526}}</ref>

==== Undiscovered ultralight particles ====
{{main|Axion}}

Axions are hypothetical elementary particles originally theorized in 1978 independently by [[Frank Wilczek]] and [[Steven Weinberg]] as the [[Goldstone boson]] of [[Peccei–Quinn theory]], which had been proposed in 1977 to solve the [[strong CP problem]] in [[quantum chromodynamics]] (QCD). QCD effects produce an effective periodic potential in which the axion field moves.<ref name="peccei2008">{{cite book|last=Peccei | first=R. D. | title=Axions: Theory, Cosmology, and Experimental Searches <!-- leave subtitle --> |year=2008 |chapter=The Strong CP Problem and Axions |editor1-last=Kuster |editor1-first=Markus |editor2-last=Raffelt |editor2-first=Georg |editor3-last=Beltrán |editor3-first=Berta |series=Lecture Notes in Physics |volume=741 |pages=3–17 |arxiv=hep-ph/0607268 |doi=10.1007/978-3-540-73518-2_1 |isbn=978-3-540-73517-5|s2cid=119482294 }}</ref> Expanding the potential about one of its minima, one finds that the product of the axion mass with the axion decay constant is determined by the topological susceptibility of the QCD vacuum. An axion with mass that is much less than 60&nbsp;keV/''c''<sup>2</sup> is long-lived and weakly interacting: a perfect dark matter candidate.

The oscillations of the axion field about the minimum of the effective potential, the so-called misalignment mechanism, generate a cosmological population of cold axions with an abundance depending on the mass of the axion.<ref name="auto">{{cite journal |last1=Preskill |first1=J. |author1-link=John Preskill |last2=Wise |first2=M. |author2-link=Mark B. Wise |last3=Wilczek |first3=F. |author3-link=Frank Wilczek |date=6 January 1983 |journal=Physics Letters&nbsp;B |volume=120 |issue=1–3 |pages=127–132 |title=Cosmology of the invisible axion |doi=10.1016/0370-2693(83)90637-8 |bibcode=1983PhLB..120..127P |url=http://www.theory.caltech.edu/~preskill/pubs/preskill-1983-axion.pdf |citeseerx=10.1.1.147.8685 }}</ref><ref name="A cosmological bound on the invisib">{{cite journal |last1=Abbott |first1=L. |last2=Sikivie |first2=P. |year=1983 |journal=Physics Letters&nbsp;B |volume=120 |issue=1–3 |pages=133–136 |title=A cosmological bound on the invisible axion |bibcode=1983PhLB..120..133A |doi=10.1016/0370-2693(83)90638-X |citeseerx=10.1.1.362.5088}}</ref><ref name="The not-so-harmless axion">{{cite journal |last1=Dine |first1=M. |last2=Fischler |first2=W. |year=1983 |journal=Physics Letters B |volume=120 |issue=1–3 |pages=137–141 |title=The not-so-harmless axion |doi=10.1016/0370-2693(83)90639-1 |bibcode=1983PhLB..120..137D}}</ref> With a mass above 5&nbsp;[[electron-volt|μeV/{{mvar|c}}<sup>2</sup>]] ({{10^|−11}} times the [[electron mass]]) axions could account for dark matter, and thus be both a dark-matter candidate and a solution to the strong CP problem. If inflation occurs at a low scale and lasts sufficiently long, the axion mass can be as low as 1&nbsp;peV/''c''<sup>2</sup>.<ref>{{cite journal |last1=di Luzio |first1=L. |last2=Nardi |first2=E. |last3=Giannotti |first3=M. |last4=Visinelli |first4=L. |date=25 July 2020 |journal=Physics Reports |volume=870 |pages=1–117 |title=The landscape of QCD axion models |bibcode=2020PhR...870....1D |doi=10.1016/j.physrep.2020.06.002 |arxiv=2003.01100 |s2cid=211678181 }}</ref><ref>{{cite journal |last1=Graham |first1=Peter W. |last2=Scherlis |first2=Adam |title=Stochastic axion scenario |journal=Physical Review D |date=9 August 2018 |volume=98 |issue=3 |page=035017 |doi=10.1103/PhysRevD.98.035017 |arxiv=1805.07362 |bibcode=2018PhRvD..98c5017G |s2cid=119432896 }}</ref><ref>{{cite journal |last1=Takahashi |first1=Fuminobu |last2=Yin |first2=Wen |last3=Guth |first3=Alan H. |title=The QCD Axion Window and Low Scale Inflation |journal=Physical Review D |date=31 July 2018 |volume=98 |issue=1 |pages=015042 |doi=10.1103/PhysRevD.98.015042 |arxiv=1805.08763 |bibcode=2018PhRvD..98a5042T |s2cid=54584447 }}</ref>

Because axions have extremely low mass, their [[de Broglie wavelength]] is very large, in turn meaning that quantum effects could help resolve the small-scale problems of the [[Lambda-CDM]] model. A single ultralight axion with a decay constant at the [[grand unified theory]] scale provides the correct relic density without fine-tuning.<ref>{{cite journal |last=Marsh |first=David J.E. |date=2016 |title=Axion cosmology |journal=Physics Reports |language=en |volume=643 |pages=1–79 |doi=10.1016/j.physrep.2016.06.005 |arxiv=1510.07633|bibcode=2016PhR...643....1M |s2cid=119264863 }}</ref>

Axions as a dark matter candidate have gained in popularity in recent years, because of the non-detection of WIMPS.<ref>{{cite web|url=https://physicsworld.com/a/dark-matters-secret-identity-wimps-or-axions/?form=MG0AV3|title=Dark matter's secret identity: WIMPs or axions?|publisher=Physics World|date = 25 June 2024}}</ref>

==== Primordial black holes ====
{{main|Primordial black hole}}

Primordial black holes are hypothetical [[black hole]]s that formed soon after the [[Big Bang]]. In the [[inflationary era]] and early [[Scale factor (cosmology)#Radiation-dominated era|radiation-dominated]] universe, extremely dense pockets of [[Subatomic particle|subatomic matter]] may have been tightly packed to the point of [[gravitational collapse]], creating primordial black holes without the [[supernova]] compression typically needed to make black holes today. Because the creation of primordial black holes would pre-date the first stars, they are not limited to the narrow mass range of [[stellar black hole]]s and also not classified as baryonic dark matter.

The idea that black holes could form in the early universe was first suggested by [[Yakov Zeldovich]] and [[Igor Dmitriyevich Novikov]] in 1967, and independently by [[Stephen Hawking]] in 1971. It quickly became clear that such black holes might account for at least part of dark matter. Primordial black holes as a dark matter candidate has the major advantage that it is based on a well-understood theory ([[General Relativity]]) and objects ([[black holes]]) that are already known to exist. However, producing primordial black holes requires exotic [[cosmic inflation]] or physics beyond the [[standard model of particle physics]],<ref>{{cite journal|last1=Villanueva-Domingo|first1=Pablo|last2=Mena|first2=Olga|last3=Palomares-Ruiz|first3=Sergio|title=A Brief Review on Primordial Black Holes as Dark Matter|journal=Frontiers in Astronomy and Space Sciences|date=2021 |volume=8 |page=87 |doi=10.3389/fspas.2021.681084|doi-access=free |arxiv=2103.12087 |bibcode=2021FrASS...8...87V }}</ref> and might also require fine-tuning.<ref>{{cite arXiv |last1= Carr|first1= Bernard J.|last2=Green|first2=Anne M.|date= June 2024|title= The History of Primordial Black Holes|eprint= 2406.05736v1|class=astro-ph.CO}}</ref> Primordial black holes can also span nearly the entire possible mass range, from atom-sized to supermassive.

The idea that primordial black holes make up dark matter gained prominence in 2015<ref>
{{cite journal
 |last1=Cho |first1=Adrian
 |date=9 February 2017
 |title=Is dark matter made of black holes?
 |journal=[[Science (journal)|Science]]
 |doi=10.1126/science.aal0721
}}
</ref> following results of [[gravitational wave]] measurements which detected the merger of intermediate-mass black holes. Black holes with about 30&nbsp;solar masses are not predicted to form by either stellar collapse (typically less than 15&nbsp;solar masses) or by the merger of black holes in galactic centers (millions or billions of solar masses), which suggests that the detected black holes might be primordial. A later survey of about a thousand supernovae detected no gravitational lensing events, when about eight would be expected if intermediate-mass primordial black holes above a certain mass range accounted for over 60% of dark matter.<ref>
{{cite news
 |title=Black holes can't explain dark matter
 |date=18 October 2018
 |magazine=[[Astronomy (magazine)|Astronomy]]
 |via=astronomy.com
 |url=http://astronomy.com/news/2018/10/can-black-holes-explain-dark-matter
 |access-date=7 January 2019
}}
</ref> However, that study assumed that all black holes have the same or similar mass to the LIGO/Virgo mass range, which might not be the case (as suggested by subsequent [[James Webb Space Telescope]] observations).<ref>
{{cite journal
 |last1=Zumalacárregui |first1=Miguel
 |last2=Seljak         |first2=Uroš
 |title=Limits on Stellar-Mass Compact Objects as Dark Matter from Gravitational Lensing of Type Ia Supernovae
 |journal=[[Physical Review Letters]]
 |date=1 October 2018
 |volume=121 |issue=14 |page=141101
 |doi=10.1103/PhysRevLett.121.141101  |pmid=30339429
 |arxiv=1712.02240  |bibcode=2018PhRvL.121n1101Z
 |s2cid=53009603
 |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.141101
 |access-date=17 August 2023
}}
</ref><ref name=jwst/>

The possibility that atom-sized primordial black holes account for a significant fraction of dark matter was ruled out by measurements of positron and electron fluxes outside the Sun's heliosphere by the ''[[Voyager 1|Voyager&nbsp;1]]'' spacecraft. Tiny black holes are theorized to emit [[Hawking radiation]]. However the detected fluxes were too low and did not have the expected energy spectrum, suggesting that tiny primordial black holes are not widespread enough to account for dark matter.<ref>
{{cite news
 |title=Aging Voyager&nbsp;1 spacecraft undermines idea that dark matter is tiny black holes
 |date=9 January 2019
 |journal=[[Science (journal)|Science]]
 |via=sciencemag.org
 |url=https://www.science.org/content/article/aging-voyager-1-spacecraft-undermines-idea-dark-matter-tiny-black-holes
 |access-date=10 January 2019
}}
</ref> Nonetheless, research and theories proposing dense dark matter accounts for dark matter continue as of 2018, including approaches to dark matter cooling,<ref>
{{cite news
 |first=Shannon |last=Hall
 |date=5 February 2018 
 |title=There could be entire stars and planets made out of dark matter
 |magazine=[[New Scientist]]
 |url=https://www.newscientist.com/article/2160305-there-could-be-entire-stars-and-planets-made-out-of-dark-matter/
}}
</ref><ref>
{{cite journal
 |last1=Buckley  |first1=Matthew R.
 |last2=Difranzo |first2=Anthony
 |year=2018
 |title=Collapsed dark matter structures
 |journal=[[Physical Review Letters]]
 |volume=120 |issue=5 |page=051102
 |arxiv=1707.03829  |bibcode=2018PhRvL.120e1102B
 |doi=10.1103/PhysRevLett.120.051102
 |pmid=29481169  |s2cid=3757868
}}
</ref> and the question remains unsettled. In 2019, the lack of microlensing effects in the observation of Andromeda suggests that tiny black holes do not exist.<ref>
{{cite journal
 |last=Niikura |first=Hiroko
 |date=1 April 2019
 |title=Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations
 |journal=[[Nature Astronomy]]
 |volume=3 |issue=6 |pages=524–534
 |doi=10.1038/s41550-019-0723-1  |s2cid=118986293 
 |bibcode=2019NatAs...3..524N  |arxiv=1701.02151 
}}
</ref>

Nonetheless, there still exists a largely unconstrained mass range smaller than that which can be limited by optical microlensing observations, where primordial black holes may account for all dark matter.<ref>
{{cite journal
 |last1=Katz       |first1=Andrey        |last2=Kopp |first2=Joachim
 |last3=Sibiryakov |first3=Sergey        |last4=Xue  |first4=Wei
 |date=5 December 2018 
 |title=Femtolensing by dark matter revisited
 |journal=Journal of Cosmology and Astroparticle Physics
 |volume=2018 |issue=12 |page=005
 |doi=10.1088/1475-7516/2018/12/005  |issn=1475-7516
 |bibcode=2018JCAP...12..005K  |arxiv=1807.11495
 |s2cid=119215426
 |url=http://stacks.iop.org/1475-7516/2018/i=12/a=005?key=crossref.dcb1e788286d8d75d0c7e7f35fe588eb
}}
</ref><ref>
{{cite journal
 |last1=Montero-Camacho |first1=Paulo        |last2=Fang  |first2=Xiao
 |last3=Vasquez  |first3=Gabriel             |last4=Silva |first4=Makana
 |last5=Hirata   |first5=Christopher M.
 |date=23 August 2019
 |title=Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates
 |journal=[[Journal of Cosmology and Astroparticle Physics]]
 |volume=2019 |issue=8 |page=031
 |doi=10.1088/1475-7516/2019/08/031
 |issn=1475-7516  |bibcode=2019JCAP...08..031M
 |arxiv=1906.05950  |s2cid=189897766
}}
</ref>

=== Modified gravity ===
{{Further|Alternatives to general relativity}}

The last major possibility is that general relativity, the theory underpinning modern cosmology, is incorrect. General relativity is well-tested on Solar System scales, but its validity on galactic or cosmological scales has not been well proven.<ref>{{cite book | author = Peebles, P. J. E.| date= December 2004<!--  | title=Testing general relativity on the scales of cosmology--> | arxiv= astro-ph/0410284|bibcode = 2005grg..conf..106P |doi = 10.1142/9789812701688_0010  | isbn = 978-981-256-424-5 | pages = 106–117 | chapter= Probing General Relativity on the Scales of Cosmology| title= General Relativity and Gravitation| s2cid= 1700265}}</ref> A suitable modification to general relativity can conceivably eliminate the need for dark matter. The best-known theories of this class are [[Modified Newtonian dynamics|MOND]] and its relativistic generalization [[tensor–vector–scalar gravity]] (TeVeS),<ref>For a review, see: {{cite journal |title=The failures of the Standard Model of Cosmology require a new paradigm |author1=Kroupa, Pavel |display-authors=etal |journal=International Journal of Modern Physics D |date=December 2012 |volume=21 |issue=4 |page=1230003 |doi=10.1142/S0218271812300030 |arxiv=1301.3907 |bibcode=2012IJMPD..2130003K|s2cid=118461811 }}</ref> [[f(R) gravity]],<ref>For a review, see: {{cite journal |title=The dark matter problem from f(R) gravity viewpoint |author=Salvatore Capozziello |author2=Mariafelicia De Laurentis |journal=Annalen der Physik |date=October 2012 |volume=524 |issue=9–10 |page=545 |doi=10.1002/andp.201200109 |bibcode=2012AnP...524..545C |doi-access=free }}</ref> [[negative mass]], [[dark fluid]],<ref>{{cite web|url= https://www.ox.ac.uk/news/2018-12-05-bringing-balance-universe |title=Bringing balance to the Universe |date=5 December 2018 |publisher=University of Oxford}}</ref><ref>{{cite web|url= https://phys.org/news/2018-12-universe-theory-percent-cosmos.html |title=Bringing balance to the universe: New theory could explain missing 95 percent of the cosmos |publisher=Phys.Org}}</ref><ref name="Farnes">{{cite journal |last=Farnes |first=J. S. |year=2018 |title=A Unifying Theory of Dark Energy and Dark Matter: Negative Masses and Matter Creation within a Modified ΛCDM Framework |journal=Astronomy & Astrophysics |volume=620 |page=A92 |arxiv=1712.07962 |bibcode=2018A&A...620A..92F |doi=10.1051/0004-6361/201832898 |s2cid=53600834}}</ref> and [[entropic gravity]].<ref name="physorgnewtheory">{{cite news |title=New theory of gravity might explain dark matter |url=https://phys.org/news/2016-11-theory-gravity-dark.html |website=phys.org |date=November 2016}}</ref> [[Alternatives to general relativity|Alternative theories]] abound.<ref>{{cite journal |title=Alternatives to dark matter and dark energy |first=Phillip D. |last=Mannheim |journal=Progress in Particle and Nuclear Physics |volume=56 |issue=2 |pages=340–445 |doi=10.1016/j.ppnp.2005.08.001 |arxiv=astro-ph/0505266 |date=April 2006 |bibcode=2006PrPNP..56..340M |s2cid=14024934 }}</ref><ref>{{cite journal |title=Beyond the Cosmological Standard Model |first1=Austin |last1=Joyce |display-authors=etal |journal=Physics Reports |date=March 2015 |volume=568 |pages=1–98 |doi=10.1016/j.physrep.2014.12.002 |arxiv=1407.0059 |bibcode=2015PhR...568....1J|s2cid=119187526 }}</ref>

A problem with modifying gravity is that observational evidence for dark matter – let alone general relativity – comes from so many independent approaches (see the "observational evidence" section above). Explaining any individual observation is possible but explaining all of them in the absence of dark matter is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a 2016 test of gravitational lensing in entropic gravity<ref>{{cite news |url=http://phys.org/news/2016-12-verlinde-theory-gravity.html |title=Verlinde's new theory of gravity passes first test |date=16 December 2016}}</ref><ref>{{cite journal |title=First test of Verlinde's theory of Emergent Gravity using Weak Gravitational Lensing measurements |first1=Margot M. |last1=Brouwer |display-authors=etal |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=466 |issue=3 |date=April 2017 |doi=10.1093/mnras/stw3192 |arxiv=1612.03034 |pages=2547–2559 |doi-access=free |bibcode=2017MNRAS.466.2547B|s2cid=18916375 }}</ref><ref>{{cite web |url=https://www.newscientist.com/article/2116446-first-test-of-rival-to-einsteins-gravity-kills-off-dark-matter/ |title=First test of rival to Einstein's gravity kills off dark matter |date=15 December 2016 |access-date=20 February 2017}}</ref> and a 2020 measurement of a unique MOND effect.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2020/12/201216155158.htm|title=Unique prediction of 'modified gravity' challenges dark matter|publisher=ScienceDaily|date=16 December 2020|access-date=14 January 2021}}</ref><ref>{{cite journal|title=Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies|first1=Kyu-Hyun|last1=Chae|display-authors=et al|journal=[[Astrophysical Journal]]|volume=904|date=20 November 2020|issue=1|page=51|doi=10.3847/1538-4357/abbb96|arxiv=2009.11525|bibcode=2020ApJ...904...51C|s2cid=221879077 |doi-access=free }}</ref>

The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter present in the universe.<ref name="CarrollTrialogue">{{cite web |author=Carroll |first=Sean |author-link=Sean M. Carroll |date=9 May 2012 |title=Dark matter vs. modified gravity: A trialogue |url=http://www.preposterousuniverse.com/blog/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/ |access-date=14 February 2017}}</ref>