The dark side of the universe: from Zwicky to accelerated expansion

More than 60 years ago Zwicky made the case that the great clusters of galaxies are held together by the gravitational force of unseen (dark) matter. Today, the case is stronger and more precise: dark, nonbaryonic matter accounts for 30±7% of the critical mass density, with baryons (most of which are dark) contributing only 4.5±1% of the critical density. The large-scale structure that exists in the universe indicates that the bulk of the nonbaryonic dark matter must be cold (slowly moving particles). The SuperKamiokande detection of neutrino oscillations shows that particle dark matter, in the form of massive neutrinos, actually exists and accounts for as much mass as bright stars. An important threshold has been crossed; particle dark matter is no longer hypothetical. Over the past few years a case has developed for dark energy. This dark, relativistic component contributes about 80±20% of the critical density and is characterized by very negative pressure (pX<−0.6ρX). Consistent with this picture of dark energy and dark matter are measurements of CMB anisotropy that indicate that matter and energy together account for the critical density (within 10%). Fundamental physics beyond the standard model is implicated in both the dark matter and dark energy puzzles: new fundamental particles (e.g., axion or neutralino) and new forms of relativistic energy (e.g., vacuum energy or a light scalar field). Dark matter and dark energy are central issues in both cosmology and particle physics. Over the next two decades a flood of precision cosmological observations and laboratory experiments will shed light on the dark side of the universe. As they do they will advance our understanding of both the universe and the laws of physics that govern it.