Cluster of galaxies are the largest and massive structure in universe which are gravitationally bound together.Each cluster of galaxy contain 10 to 1000 galaxies.Galaxies in cluster have a very large peculiar velocity of 800-1000 km/s to be virialized in cluster.
Beside the galaxies it is found that there is a hot plasma gas in the clusters with the name of ICM "Inter cluster mass" which is approximately 2 times massive that the baryonic matter in galaxies.
The mysterious aspect of clusters is that if this structures are in hydrostatic equilibrium they need Dark matter with mass of 6 times more than baryonic matter in cluster.
So in this case there are three main constituents in each cluster:
1)5% galaxies 2)10% X-ray emitting hot gas 3)Dark matter.
It seems that the dark matter ,galaxies and ICM has approximately the same distribution in clusters.
Cluster of galaxies are the most massive and lately formed structure in the hierarchical structure formation model.They masses vary from 10^14 to 10^ 15 solar masses and the diameter of this clusters are from 2 to 10 Mpc.
The nearby important clusters are Virgo cluster, Hercules cluster and Coma cluster.
The galaxy clusters are a rich regions for observational cosmology and consequently theoretical cosmology.
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