The name given to a large swarm of galaxies connected to one another through mutual gravitational attraction. Clusters of galaxies can range from dozens to thousands of members, covering regions of space tens of millions of light years in diameter. The lower limit for membership of a cluster is about fifty galaxies; below this level, a collection of connected galaxies is usually known simply as a 'group'. Clusters themselves can combine to form even more immense structures known as superclusters.
Structure and Composition
In visible light, galaxy clusters appear as collections of individual galaxies, but in fact these visible galaxies form only a fraction of the mass of a typical cluster. Between the galaxies of the cluster is a region of gas known as the intracluster medium or ICM, accounting for about ten times the mass of the galaxies themselves. Though transparent to visible light, the energy of this medium is detectable in the X-ray spectrum.
Far more significant than either of these elements is dark matter, accounting for ten times again the combined mass of the galaxies and the ICM. This dark matter is currently impossible to detect by any direct means, but its gravitational effects on the cluster as a whole reveal its presence.
A swarm of galaxies within a galaxy cluster. This example shows a view of some of the two hundred galaxies that make up the HerculesGalaxy Cluster, Abell 2151, which lies about five hundred million light years from the Milky Way. Imagery provided by Aladin sky atlas