One of the most fundamental questions in cosmology is: How much matter is there in the Universe and how is it distributed? Here I review several independent measures—including those utilizing clusters of galaxies—that show that the mass-density of the Universe is only ∼ 20% of the critical density. Recent measurements of the mass-to-light function—from galaxies, to groups, clusters, and superclusters—provide a powerful new measure of the universal density. The results reveal a low density of 0.16 ± 0.05 the critical density. The observations suggest that, on average, the mass distribution follows the light distribution on large scales. The results, combined with recent observations of high redshift supernovae and the spectrum of the CMB anisotropy, suggest a Universe that has low density (Ωm ≃ 0.2), is flat, and is dominated by dark energy.

Introduction

Theoretical arguments based on standard models of inflation, as well as on the demand of no “fine tuning” of cosmological parameters, predict a flat universe with the critical density needed to just halt its expansion (1.9 × 10–29 h2 g cm–3). Observations, however, reveal only a small fraction of the critical density, even when all the unseen dark matter in galaxy halos and clusters of galaxies is included. There is no reliable indication that the matter needed to close the universe does in fact exist. Here I review several independent observations of clusters of galaxies which indicate, independently, that the mass density of the universe is sub-critical.