Clusters of galaxies are self-gravitating systems of mass \sim 10 ^ { 14 } -10 ^ { 15 } h ^ { -1 } M _ { \odot } and size \sim 1 - 3 h ^ { -1 } Mpc .
Their mass budget consists of dark matter ( \sim 80 \% , on average ) , hot diffuse intracluster plasma ( \lesssim 20 \% ) and a small fraction of stars , dust , and cold gas , mostly locked in galaxies .
In most clusters , scaling relations between their properties , like mass , galaxy velocity dispersion , X-ray luminosity and temperature , testify that the cluster components are in approximate dynamical equilibrium within the cluster gravitational potential well .
However , spatially inhomogeneous thermal and non-thermal emission of the intracluster medium ( ICM ) , observed in some clusters in the X-ray and radio bands , and the kinematic and morphological segregation of galaxies are a signature of non-gravitational processes , ongoing cluster merging and interactions .
Both the fraction of clusters with these features , and the correlation between the dynamical and morphological properties of irregular clusters and the surrounding large-scale structure increase with redshift .

In the current bottom-up scenario for the formation of cosmic structure , where tiny fluctuations of the otherwise homogeneous primordial density field are amplified by gravity , clusters are the most massive nodes of the filamentary large-scale structure of the cosmic web and form by anisotropic and episodic accretion of mass , in agreement with most of the observational evidence .
In this model of the universe dominated by cold dark matter , at the present time most baryons are expected to be in a diffuse component rather than in stars and galaxies ; moreover , \sim 50 \% of this diffuse component has temperature \sim 0.01 - 1 keV and permeates the filamentary distribution of the dark matter .
The temperature of this Warm-Hot Intergalactic Medium ( WHIM ) increases with the local density and its search in the outer regions of clusters and lower density regions has been the quest of much recent observational effort .

Over the last thirty years , an impressive coherent picture of the formation and evolution of cosmic structures has emerged from the intense interplay between observations , theory and numerical experiments .
Future efforts will continue to test whether this picture keeps being valid , needs corrections or suffers dramatic failures in its predictive power .