Cosmos The Cosmic Journey

DARK MATTER

Dark matter is a hypothetical kind of matterthat cannot be seen with telescopes but accounts for most of the matter in theuniverse. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and thelarge-scale structure of the universe. It has not been detected directly, making it one of the greatest mysteries in modernastrophysics.
Much of the evidence for dark matter comes from the study of the motions of galaxies.[27]Many of these appear to be fairly uniform, so by the virial theorem, the total kinetic energyshould be half the total gravitational binding energy of the galaxies. Observationally, however, the total kinetic energy is found to be much greater: in particular, assuming thegravitational mass is due to only the visible matter of the galaxy, stars far from the center of galaxies have much higher velocities than predicted by the virial theorem. Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, show the well known phenomenology that cannot be explained by only the visible matter. Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this. Galaxies show signs of being composed largely of a roughly spherically symmetric, centrally concentrated halo of dark matter with the visible matter concentrated in a disc at the center. Low surface brightness dwarf galaxies are important sources of information for studying dark matter, as they have an uncommonly low ratio of visible matter to dark matter, and have few bright stars at the center which would otherwise impair observations of the rotation curve of outlying stars.