Clusters of galaxies (see extragalactic systems) are groups of tens to thousands of galaxies that are bound together gravitationally.
The Milky Way galaxy (see Galaxy, The) is a member of a small cluster known as the Local Group of galaxies. Regular clusters, such as the Coma cluster, exhibit symmetry and a high central density; irregular clusters, such as the Virgo cluster of galaxies, are unsymmetrical and are not centrally concentrated. Thousands of clusters of galaxies have been identified and cataloged.