Galaxies are not evenly distributed throughout the universe. A few float alone like isolated islands in space, but most are grouped in formations termed galactic clusters. These formations should not be confused with stellar clusters, which are globular clusters of stars that exist within a galaxy. The size of galactic clusters varies enormously, with some clusters containing only a dozen or so members, and others containing as many as 10,000. Clusters containing less than 50 galaxies are called "poor" clusters, while those with more than 50 galaxies are considered "rich." Galactic clusters themselves are part of larger clusters of clusters, termed superclusters. It is surmised that even clusters of superclusters are possible.
Our own galaxy, the Milky Way, and our closest galactic neighbors are part of a cluster called the Local Group. The existence of the Local Group was first discovered by the astronomer Edwin Hubble. He coined the term "Local Group" in his 1936 book The Realm of the Nebulae. The Local Group has around 30 members and is typical in terms of the types of galaxies it contains. There are three large spiral galaxies: Andromeda, the largest galaxy in the group; the Milky Way, the second largest galaxy; and the Triangulum Spiral, the third largest. There are also four medium-sized irregular galaxies, including the Large Cloud of Magellan and the Small Cloud of Magellan, which are companions of our own galaxy. There are four regular elliptical galaxies, including two that are companions of the galaxy Andromeda. The remainder are dwarf ellipticals. Other than our own galaxy, only Andromeda and the Clouds of Magellan can be seen with the naked eye, and the Clouds are visible only from the Southern Hemisphere.
[A] At one time, another large galaxy, Maffei I, was believed to be part of the Local Group. [B] It was discovered in the late 1960s by Italian astronomer Paolo Maffei, who used a radio telescope to "see" through this interstellar mist. [C] More recent studies, however, indicate that it is actually part of a neighboring galactic cluster. [D]
In the vicinity of the Local Group are several small clusters, each containing around 12 members. The nearest cluster that is rich in members is the Virgo Cluster, which contains thousands of galaxies of all types. The Local Supercluster, which includes the Local Group and the small neighboring clusters, is dominated by the Virgo Cluster. Other notable rich clusters include the Coma Cluster, the Hercules Cluster, and the Perseus Cluster. These clusters contain vast amounts of hot gases that emit x-rays, a phenomenon characteristic of rich clusters, and in all of them, the majority of galaxies are concentrated in their centers, another common feature of clusters with many members.
The existence of galactic clusters presented a riddle to scientists for many years--the "missing mass" problem. It was presumed that clusters were held together by the gravity generated by their members. However, measurements showed that the galaxies do not have nearly enough mass to explain their apparent stability. Why don't these clusters disintegrate? One theory is that there is some kind of attractive force in addition to gravity that holds clusters together. However, today, most scientists believe in the "dark matter" theory first suggested by Swiss astrophysicist Fritz Zwicky. According to this theory, galactic clusters contain great amounts of "dark matter" that cannot be directly observed but that generates gravitational pull. In a typical cluster perhaps only 15% of the total mass is visible. About 5% is in the form of galaxies and 10% is in the form of hot x-ray-emitting gases, while the remainder is dark matter. The composition of dark matter is unknown, but it might include cold gases, dust, burnt-out stars, perhaps black holes, and maybe even more mysterious components.
