Galaxy cluster

Galaxy clusters are large collections of galaxies. They consist of hundreds of galaxies and galaxy groups, bound together by gravity. Galaxy clusters are much larger than galaxy groups, like our Local Group. 

Clusters of galaxies should not be confused with either star clusters, which are inside galaxies, or with globular clusters, which usually orbit galaxies.

Notable galaxy clusters in the relatively nearby universe include the Virgo Cluster (which includes our Local Group), Fornax Cluster, Hercules Cluster, and the Coma Cluster. 

Still larger than galaxy clusters are superclusters. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma Cluster, is massive enough to affect the local expansion of the universe (Hubble's law).

Notable galaxy clusters in the distant, high-redshift universe include SPT-CL J0546-5345, the most massive galaxy cluster ever found in the early universe.

Basic properties
Galaxy clusters typically have the following properties.
 They contain 50 to 1,000 galaxies, hot X-ray emitting gas and large amounts of dark matter. Details are described in the "Composition" section.
 The distribution of these three components is approximately the same in the cluster.
 They have total masses of 1014 to 1015 solar masses.
 They typically have a diameter from 2 to 10 megaparsecs.
 The spread of velocities for the individual galaxies is about 800–1000 km/s.
 The intracluster medium or ICM has gas between the galaxies with a temperature of 7-9 keV.

Composition
There are three main components of a galaxy cluster. They are:
 Name of the components Mass fraction Description Galaxies 1% In optical observations only galaxies are visible Gas between galaxies, inside the cluster 9% Plasma between the galaxies at high temperature – emit x-ray radiation Dark matter 90% Most massive component, cannot be seen, inferred by gravitational interactions

 Scale of things 
Where ">" means "contains":

Universe > observable universe > very large-scale structures > superclusters > galaxy clusters > galaxy groups > galaxies > star clusters > stars

 References