Editing Galaxy cluster (section)

{{short description|Structure made up of a gravitationally-bound aggregation of hundreds of galaxies}}[[File:BoRG-58.jpg|thumb|right|300px|Composite image of five galaxies clustered together just 600 million years after the Universe's birth<ref name="Hubble protocluster">{{cite news|title=Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen|url=http://www.spacetelescope.org/news/heic1201/|access-date=13 January 2015|newspaper=ESA/Hubble Press Release}}</ref>]]

A '''galaxy cluster''', or a '''cluster of galaxies''', is a structure that consists of anywhere from hundreds to thousands of [[galaxy|galaxies]] that are bound together by [[gravity]],<ref name="Hubble protocluster" />  with typical masses ranging from 10<sup>14</sup> to 10<sup>15</sup> [[solar mass]]es. Clusters consist of galaxies, heated gas, and dark matter.<ref name=":0" /> They are the second-largest known [[gravitational binding energy|gravitationally bound]] structures in the [[universe]] after [[supercluster]]s. They were believed to be the [[Observable universe#Large-scale structure|largest known structures]] in the universe until the 1980s, when [[supercluster]]s were discovered.<ref name="Kravtsov2012">{{Cite journal |last1=Kravtsov|first1=A. V. |last2=Borgani|first2=S. |doi=10.1146/annurev-astro-081811-125502 |title=Formation of Galaxy Clusters | journal=[[Annual Review of Astronomy and Astrophysics]] |volume=50 |pages=353–409 |year=2012 |arxiv=1205.5556 |bibcode=2012ARA&A..50..353K |s2cid=119115331}}</ref>  Small aggregates of galaxies are referred to as [[galaxy group]]s rather than clusters of galaxies. Together, [[galaxy groups and clusters]] form superclusters.