Editing Dark matter (section)

== Dark matter aggregation and dense dark matter objects ==
If dark matter is composed of weakly interacting particles, then an obvious question is whether it can form objects equivalent to [[planet]]s, [[star]]s, or [[black hole]]s. Historically, the answer has been it cannot,{{efn|
"One widely held belief about dark matter is it cannot cool off by radiating energy. If it could, then it might bunch together and create compact objects in the same way baryonic matter forms planets, stars, and galaxies. Observations so far suggest dark matter doesn't do that – it resides only in diffuse halos ... As a result, it is extremely unlikely there are very dense objects like stars made out of entirely (or even mostly) {{nobr|dark matter." — Buckley & Difranzo (2018)<ref name=curio/>}}
}}<ref name=curio>
{{cite journal
 |last1=Buckley |first1=Matthew R.
 |last2=Difranzo |first2=Anthony
 |date=1 February 2018
 |title=Synopsis: A way to cool dark matter
 |journal=[[Physical Review Letters]]
 |volume=120  |issue=5  |page=051102
 |doi=10.1103/PhysRevLett.120.051102
 |bibcode=2018PhRvL.120e1102B  |pmid=29481169
 |arxiv=1707.03829  |s2cid=3757868
 |url=https://physics.aps.org/articles/v11/s15
 |archive-url=https://archive.today/20201026224145/https://physics.aps.org/articles/v11/s15
 |archive-date=26 October 2020
}}
</ref><ref name=cornell_ask>
{{cite web
 |title=Are there any dark stars or dark galaxies made of dark matter?
 |department=Ask an Astronomer
 |website=curious.astro.cornell.edu
 |publisher=[[Cornell University]]
 |url=http://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced
 |archive-url=https://web.archive.org/web/20150302105015/http://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced
 |archive-date=2 March 2015
}}
</ref><ref name=siegel>
{{cite magazine
 |author-link=Ethan Siegel |author=Siegel, Ethan
 |date=28 October 2016
 |title=Why doesn't dark matter form black holes?
 |magazine=[[Forbes (magazine)|Forbes]]
 |url=https://www.forbes.com/sites/startswithabang/2016/10/28/why-doesnt-dark-matter-form-black-holes/#4e5014943de1
}}
</ref> because of two factors:
; It lacks an efficient means to lose energy<ref name=curio/> 
: Ordinary matter forms dense objects because it has numerous ways to lose energy. Losing energy would be essential for object formation, because a particle that gains energy during compaction or falling "inward" under gravity, and cannot lose it any other way, will heat up and increase [[velocity]] and [[momentum]]. Dark matter appears to lack a means to lose energy, simply because it is not capable of interacting strongly in other ways except through gravity. The [[virial theorem]] suggests that such a particle would not stay bound to the gradually forming object – as the object began to form and compact, the dark matter particles within it would speed up and tend to escape.
; It lacks a diversity of interactions needed to form structures<ref name=siegel/>
: Ordinary matter interacts in many different ways, which allows the matter to form more complex structures. For example, stars form through gravity, but the particles within them interact and can emit energy in the form of [[neutrino]]s and [[electromagnetic radiation]] through [[nuclear fusion|fusion]] when they become energetic enough. [[Proton]]s and [[neutron]]s can bind via the [[strong interaction]] and then form [[atom]]s with [[electron]]s largely through [[electromagnetic interaction]]. There is no evidence that dark matter is capable of such a wide variety of interactions, since it seems to only interact through gravity (and possibly through some means no stronger than the [[weak interaction]], although until dark matter is better understood, this is only speculation).