Editing Galaxy groups and clusters (section)

{{Short description|Largest known gravitationally bound object in universe; aggregation of galaxies}}
[[File:A scattering of spiral and elliptical galaxies.jpg|thumb|290px|[[MACS J0152.5-2852]] is a massive galaxy cluster. Almost every pixel seen in the image is a [[galaxy]], each containing billions of [[Star|stars]].<ref>{{cite news |title=A scattering of spiral and elliptical galaxies |url=http://www.spacetelescope.org/images/potw1338a/ |access-date=25 September 2013 |newspaper=ESA/Hubble Picture of the Week}}</ref>]]

'''Galaxy groups and clusters''' are the largest [[Observable universe|known]] [[Gravitational binding energy|gravitationally bound]] objects to have arisen thus far in the process of cosmic structure formation.<ref>{{cite journal|doi=10.1103/revmodphys.77.207|title=Tracing cosmic evolution with clusters of galaxies|journal=Reviews of Modern Physics|volume=77|issue=1|pages=207–258|year=2005|last1=Voit|first1=G. Mark|bibcode=2005RvMP...77..207V|arxiv=astro-ph/0410173|s2cid=119465596}}</ref> They form the densest part of the [[Observable universe#Large-scale structure|large-scale structure of the Universe]]. In models for the gravitational formation of structure with [[cold dark matter]], the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called [[supercluster]]s.