Editing Dark matter (section)

=== Cosmic microwave background ===
{{Main|Cosmic microwave background}}
Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via [[Thomson scattering]]. Dark matter does not interact directly with radiation, but it does affect the cosmic microwave background (CMB) by its gravitational potential (mainly on large scales) and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the CMB.

The CMB is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The locations of these peaks depend on cosmological parameters. Matching theory to data, therefore, constrains cosmological parameters.<ref name="Wayne Hu">The details are technical. For an intermediate-level introduction, see {{cite web |author=Hu, Wayne |title=Intermediate Guide to the Acoustic Peaks and Polarization |url=http://background.uchicago.edu/~whu/intermediate/intermediate.html |year=2001}}</ref>

The CMB anisotropy was first discovered by [[Cosmic Background Explorer|COBE]] in 1992, though this had too coarse resolution to detect the acoustic peaks.
After the discovery of the first acoustic peak by the balloon-borne [[BOOMERanG]] experiment in 2000, the power spectrum was precisely observed by [[WMAP]] in 2003–2012, and even more precisely by the [[Planck spacecraft|''Planck'' spacecraft]] in 2013–2015. The results support the Lambda-CDM model.<ref name="Hinshaw2009">{{cite journal |last1=Hinshaw |first1=G. |display-authors=etal |year=2009 |title=Five-year Wilkinson microwave anisotropy probe (WMAP) observations: Data processing, sky maps, and basic results |journal=The Astrophysical Journal Supplement |volume=180 |issue=2 |pages=225–245 |arxiv=0803.0732 |bibcode=2009ApJS..180..225H |doi=10.1088/0067-0049/180/2/225|s2cid=3629998 }}</ref><ref name="Planck15" />

The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is well fitted by the [[Lambda-CDM model]],<ref name="Planck15">{{cite journal |last1=Ade |first1=P.A.R. |display-authors=etal |title=Planck 2015 results. XIII. Cosmological parameters |journal=Astron. Astrophys. |year=2016 |volume=594 |issue=13 |page=A13 |doi=10.1051/0004-6361/201525830 |bibcode=2016A&A...594A..13P |arxiv=1502.01589 |s2cid=119262962 }}</ref> but difficult to reproduce with any competing model such as [[modified Newtonian dynamics]] (MOND).<ref>{{cite journal |last1=Skordis |first1=C. |display-authors=etal |title=Large scale structure in Bekenstein's theory of relativistic modified Newtonian dynamics |journal=Phys. Rev. Lett. |year=2006 |volume=96 |issue=1 |page=011301 |doi=10.1103/PhysRevLett.96.011301 |pmid=16486433 |bibcode=2006PhRvL..96a1301S |arxiv=astro-ph/0505519|s2cid=46508316 }}</ref>