Editing Dark matter (section)

=== 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>