galaxy cluster

Composite image of BoRG-58, a group of 5 galaxies clustered together just 600 million years after the Universe's birth

A galaxy cluster, or a 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 10 to 10 solar masses (M☉). Clusters consist of galaxies, heated gas, and dark matter. They are the biggest known gravitationally bound structures in the universe. They were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. Together, galaxy groups and clusters form superclusters.