A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity  with typical masses ranging from 1014–1015 solar masses. They are the largest known gravitationally bound structures in the universe and were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered.  One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with star clusters, such as galactic clusters—also known as open clusters—which are structures of stars within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.
Notable galaxy clusters in the relatively nearby Universe include the Virgo Cluster, Fornax Cluster, Hercules Cluster, and the Coma Cluster. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma Cluster, is massive enough to affect the local expansion of the Universe. Notable galaxy clusters in the distant, high-redshift Universe include SPT-CL J0546-5345 and SPT-CL J2106-5844, the most massive galaxy clusters found in the early Universe. In the last few decades, they are also found to be relevant sites of particle acceleration, a feature that has been discovered by observing non-thermal diffuse radio emissions, such as radio halos and radio relics. Using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters.
Galaxy clusters typically have the following properties:
- They contain 100 to 1,000 galaxies, hot X-ray emitting gas and large amounts of dark matter.  Details are described in the "Composition" section.
- The distribution of the three components is approximately the same in the cluster.
- They have total masses of 1014 to 1015 solar masses.
- They typically have a diameter from 1 to 5 Mpc (see 1023 m for distance comparisons).
- The spread of velocities for the individual galaxies is about 800–1000 km/s.
There are three main components of a galaxy cluster. They are tabulated below:[ citation needed]
|Name of the components||Mass fraction||Description|
|Galaxies||1%||In optical observations, only galaxies are visible|
|Intergalactic gas in intracluster medium||9%||Plasma between the galaxies at high temperature and emit x-ray radiation by thermal bremsstrahlung|
|Dark matter||90%||Most massive component but cannot be detected optically and is inferred through gravitational interactions|
Stars, Star clusters, Galaxies, Galaxy clusters, Super clusters
|Virgo Cluster||The nearest massive galaxy cluster|
|Norma Cluster||The cluster at the heart of the Great Attractor|
|Bullet Cluster||A cluster merger with the first observed separation between dark matter and normal matter|
|This lists some of the most notable clusters; for more clusters, see the list article.|
Galaxy cluster WHL J24.3324-8.477. 
Background galaxy has been gravitationally lensed by the intervening galaxy cluster. 
The galaxy cluster Abell 2813 (also known as ACO 2813) image from the NASA/ESA Hubble Space Telescope
|Wikimedia Commons has media related to Galaxy clusters.|
- "Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen". ESA/Hubble Press Release. Retrieved 13 January 2012.
- Kravtsov, A. V.; Borgani, S. (2012). "Formation of Galaxy Clusters". Annual Review of Astronomy and Astrophysics. 50: 353–409. arXiv: 1205.5556. Bibcode: 2012ARA&A..50..353K. doi: 10.1146/annurev-astro-081811-125502. S2CID 119115331.
- "Galaxy cluster IDCS J1426". Retrieved 11 January 2016.
- "Chandra :: Field Guide to X-ray Sources :: Groups & Clusters of Galaxies".
- Clavin, Whitney; Jenkins, Ann; Villard, Ray (7 January 2014). "NASA's Hubble and Spitzer Team up to Probe Faraway Galaxies". NASA. Retrieved 8 January 2014.
- Chou, Felecia; Weaver, Donna (16 October 2014). "RELEASE 14-283 – NASA's Hubble Finds Extremely Distant Galaxy through Cosmic Magnifying Glass". NASA. Retrieved 17 October 2014.
- "Distant and ancient". www.spacetelescope.org. Retrieved 6 May 2019.
- "Strings of homeless stars". www.spacetelescope.org. Retrieved 11 June 2018.
- "From toddlers to babies". www.spacetelescope.org. Retrieved 7 May 2018.
- "Approaching the Universe's origins". www.spacetelescope.org. Retrieved 16 April 2018.
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- "Cosmic archaeology". www.spacetelescope.org. Retrieved 24 October 2017.
- "Hubble pushed beyond limits to spot clumps of new stars in distant galaxy". www.spacetelescope.org. Retrieved 12 July 2017.
- Loff, Sarah; Dunbar, Brian (10 February 2015). "Hubble Sees A Smiling Lens". NASA. Retrieved 10 February 2015.
- "Image of the galaxy cluster SpARCS1049". Retrieved 11 September 2015.
- "Magnifying the distant Universe". ESA/Hubble Picture of the Week. Retrieved 10 April 2014.