Editing Dark matter (section)

=== Galaxy clusters ===
[[Galaxy clusters]] are particularly important for dark matter studies since their masses can be estimated in three independent ways:
* From the scatter in radial velocities of the galaxies within clusters
* From [[X-ray]]s emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster's mass profile.
* [[Gravitational lens]]ing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).
 
Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.<ref>{{cite journal |title=Cosmological Parameters from Clusters of Galaxies |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=49 |issue=1 |pages=409–470 |doi=10.1146/annurev-astro-081710-102514 |arxiv=1103.4829 |bibcode=2011ARA&A..49..409A |year=2011 |last1=Allen |first1=Steven W. |last2=Evrard |first2=August E. |last3=Mantz |first3=Adam B. |s2cid=54922695 }}</ref>