Editing Dark matter (section)

=== Bullet Cluster ===
{{Main|Bullet Cluster}}
The Bullet Cluster is the result of a recent collision of two galaxy clusters. It is of particular note because the location of the [[center of mass]] as measured by gravitational lensing is different from the location of the center of mass of visible matter. This is difficult for modified gravity theories, which generally predict lensing around visible matter, to explain.<ref>{{cite conference |url=http://cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-url=https://web.archive.org/web/20060821074820/http://www.cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-date=2006-08-21 |url-status=live |title=Dark matter and the Bullet Cluster |author1=Markevitch, M. |author2=Randall, S. |author3=Clowe, D. |author4=Gonzalez, A. |author5=Bradac, M. |name-list-style=amp |conference=36th COSPAR Scientific Assembly |date=16–23 July 2006 |location=Beijing, China}} Abstract only</ref><ref>{{cite journal |last1=Clowe |first1=Douglas |display-authors=etal |year=2006 |title=A Direct Empirical Proof of the Existence of Dark Matter |journal=The Astrophysical Journal Letters |volume=648 |issue=2 |pages=L109–L113 |arxiv=astro-ph/0608407 |doi=10.1086/508162 |bibcode=2006ApJ...648L.109C|s2cid=2897407 }}</ref><ref>{{cite web |url=https://arstechnica.com/science/2017/09/science-in-progress-did-the-bullet-cluster-withstand-scrutiny/ |title=Science-in-progress: Did the Bullet Cluster withstand scrutiny? |website=Ars Technica |author=Lee, Chris |date=21 September 2017}}</ref><ref>{{cite magazine |url=https://www.forbes.com/sites/startswithabang/2017/11/09/the-bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-physicists-think/#3032b6081738 |title=The Bullet Cluster proves dark matter exists, but not for the reason most physicists think |magazine=Forbes |author=Siegel, Ethan |author-link=Ethan Siegel |date=9 November 2017}}</ref> Standard dark matter theory however has no issue: the hot, visible gas in each cluster would be cooled and slowed down by electromagnetic interactions, while dark matter (which does not interact electromagnetically) would not. This leads to the dark matter separating from the visible gas, producing the separate lensing peak as observed.<ref>{{cite web|url=https://chandra.harvard.edu/graphics/resources/handouts/lithos/bullet_lithos.pdf|title=Bullet Cluster: Direct Proof of Dark Matter|publisher=NASA}}</ref>