A collection of galaxies that is drawn together by gravity. Galaxy clusters are the largest objects in the universe that have a gravitational pull strong enough to overcome the Hubble Expansion, and thus galaxies in a cluster to not expand away from each other.
Galaxies can swarm together to form groups and clusters of galaxies held together by their mutual gravity. X-ray observations show that these enormous systems of galaxies are filled with colossal clouds of hot gas. These clouds have temperatures as high as a hundred million degrees and contain as much mass as all the stars in the galaxies in the cluster. [More Info: Field Guide