Cosmology with clusters of galaxies

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 determining two fundamental properties of the universe: the mass-density of the universe, and the distribution of dark matter. I show that several independent methods utilizing clusters of galaxies - cluster dynamics and the mass-to-light function, baryon fraction in clusters, and cluster abundance and its evolution - all indicate the same result: a universe with a low mass-density, ∼20% of the critical density needed to halt the universal expansion, and a mass distribution that approximately traces light on large scales.