Editing Dark matter (section)

=== Structure formation ===
{{Main|Structure formation}}
[[File:Dark matter map of KiDS survey region (region G12).jpg|right|thumb|Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree Survey<ref>{{cite web |title=Dark matter may be smoother than expected – Careful study of large area of sky imaged by VST reveals intriguing result |url=https://www.eso.org/public/news/eso1642/ |access-date=8 December 2016 |website=www.eso.org}}</ref>]]Structure formation refers to the period after the [[Big Bang]] when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the [[FRW metric|Friedmann solutions]] to general relativity describe a homogeneous universe. Later, small [[Anisotropy|anisotropies]] gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure.<ref name="Jaffe">{{cite web |author=Jaffe |first=A. H. |title=Cosmology 2012: Lecture Notes |url=http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf |archive-url=https://web.archive.org/web/20160717223916/http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf |archive-date=July 17, 2016}}</ref> If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.

Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive [[potential well]] for ordinary matter collapsing later, speeding up the structure formation process.<ref name="Jaffe" /><ref>{{cite journal |author=Low |first=L. F. |date=12 October 2016 |title=Constraints on the composite photon theory |url=https://zenodo.org/record/896052 |journal=Modern Physics Letters A |volume=31 |issue=36 |page=1675002 |bibcode=2016MPLA...3175002L |doi=10.1142/S021773231675002X}}</ref>