Galaxy clusters is the name of the clusters of galaxies mutually related with gravitational force. They can be divided into regular and irregular.
Regular clusters of galaxies have a concentrated core of well-defined spherical structure. In them dominate a elliptical galaxies. They are divided according to their wealth, the number of galaxies within a zone around the center of diameter of about 2 megaparsecs, the distance called Abell radius. Typical clusters have diameters of 1-10 megaparsecs with the masses of 1*1015 masses of the Sun. In the center of the majority of regular clusters can be found a giant elliptical galaxy, sometimes with multiple nuclei.
It is believed that most galaxies formed in a process called "galactic cannibalism". Regular clusters can be seen from up to several billion light years.
An example is cluster in constellation Coma Berenices, with a diameter of about 10 million ly, and disatnce approximately 300000000 ly. The center of this cluster is one of the densest known regions on this scale in the universe.
Irregular clusters of galaxies have easily identifiable core, and they have the same range of diameters, but with much smaller mass - typically from 1*1012 to 1*1014 masses of the Sun. An example is the cluster of a constellation Virgo, with a diameter of about 7 million ly and about 50 million ly away, contains about 2500 galaxies, mostly elliptical. Galaxy Messier 87 is the largest of the 16 galaxies in this cluster which are listed in the famous Messier catalog.
The space between galaxies is filled with intergalactic medium. This medium has several components. Emission of X-rays revealed the presence of a rare hot gas. This gas involved in the mass of cluster with 10%. Gravitational interaction between galaxies is sometimes enough to shear off and throw some stars into intergalactic space. These types of galaxies and intergalactic media, giving a total of only 20% of the calculated mass of clusters. The rest is dark matter.