Galaxies tend to be gathered in clusters, which differ in the number of bright galaxies they contain and in the regularity of their appearances. The Milky Way is located in a small cluster called the Local Group.
Beyond the Local Group, other clusters of galaxies are scattered throughout space, typically separated by tens of megaparsecs. The Virgo cluster and the Coma cluster, much richer clusters than the Local Group, are among the nearest great clusters of galaxies.
The fraction of elliptical and S0 galaxies in a cluster increases as the density of the cluster increases. The dominance of these galaxies in rich clusters probably arose partly from a tendency of galaxies forming in dense regions to become clumpy and evolve into ellipticals. Collisions, which are more likely in rich clusters, also could have produced giant ellipticals and S0 galaxies while destroying spirals.
The three current theories of the formation of clusters of galaxies are the pancake model, in which clusters formed first and then fragmented to form galaxies; the hierarchical clustering model, in which clusters collected from galaxies that had already formed; and the explosion model, in which galaxies and clusters formed on dense shells of gas blasted outward by great explosions. At the present time, the hierarchical clustering model is favored because of its successes and the shortcomings of the other models.
X rays from many clusters show the presence of hot, tenuous gas between the galaxies. There is too much hot intergalactic gas for it to have come from stars and galaxies, so it must have been present in the cluster since the cluster formed.
The velocities of the galaxies in clusters are too large for the clusters to hold themselves together unless they contain about ten times more dark matter than luminous matter. The images of distant quasars and galaxies are sometimes distorted by gravitational lensing by intervening galaxies and clusters of galaxies. The distortion of these images can be used to study the distribution of matter in clusters of galaxies. The results support the conclusion that most of the matter in clusters is invisible.
Galaxy clusters are themselves grouped into superclusters. Clusters and superclusters are arranged in sheets that surround voids within which there are few, if any galaxies. A large concentration of mass (the Great Attractor) beyond the Hydra- Centaurus Supercluster is pulling the galaxies in our part of the universe toward itself.
