A galaxy cluster is a gravitationally bound collection of hundreds to thousands of galaxies. They are the largest gravitationally bound structures in the universe. The Milky Way is part of a small group called the Local Group, which contains about 50 galaxies. The nearest large cluster is the Virgo Cluster, about 54 million light-years away.

Clusters are not just collections of galaxies. They contain vast amounts of hot gas, called the intracluster medium, which emits X-rays. They also contain dark matter, which dominates their mass. The visible galaxies make up only a few percent of a cluster's total mass. The rest is gas and dark matter.

Clusters are useful for cosmology. Their abundance and distribution depend on the expansion history of the universe and the properties of dark matter and dark energy. Counting clusters at different distances tells us how the universe has evolved.

Clusters also act as gravitational lenses. Their massive dark matter halos bend the light of background galaxies, magnifying and distorting them. That effect lets astronomers study galaxies that would otherwise be too faint to see. The Hubble Space Telescope has used cluster lenses to find some of the most distant galaxies known.