XENON REU at Nevis Labs, Columbia University

It is believed that baryons, which form ordinary, luminous matter, only represent about 4% of the composition of the universe; the rest is comprised of about 73% dark energy and 23% dark matter, both of which currently have unknown sources. Dark energy explains the acceleration of the expansion of the universe, while dark matter accounts for some ”missing mass,” whose presence can be inferred from gravitational effects on visible matter. Zwicky is usually credited with proposing the modern idea of dark matter as a result of his studies of the Coma galaxy cluster in 1933 (see Figure 1). He discovered that many more times the luminous mass of the cluster is needed to keep the galaxies together. Another convincing source of evidence for dark matter is from the flat rotation curves of spiral galaxies (see Figure 2). In the late 1960s, Vera Rubin noticed that the rotational velocity of some spiral galaxies remains constant at large radii, despite the fact that the expected rotational velocity of a galactic disk falls off at large radii. However, the observed rotation curve can be explained if one sums the effects from gas, the galaxy disk, and a dark galactic halo. Other support for the concept of dark matter comes from the CMB anisotropies, supernova data, and weak gravitational lensing [1].