Rich clusters of galaxies , the largest virialized systems known , place some of the most powerful constraints on cosmology .
I discuss below the use of clusters of galaxies in addressing two fundamental questions : What is the mass-density of the universe ?
and how is the mass distributed ?
I show that several independent methods utilizing clusters of galaxies—cluster dynamics and mass-to-light ratio , baryon fractions in clusters , and cluster evolution— all indicate the same robust result : the mass-density of the universe is low , \Omega _ { m } \simeq 0.2 , and the mass approximately traces light on large scales .