Dark matter in galaxies

Current ideas on the amount, distribution, and nature of dark matter in galaxies are reviewed. Observations indicate that dark halos around most, if not all, galaxies. Recent evidence suggests that many dwarf galaxies have higher dark-matter fractions than normal galaxies as well as higher central dark-matter densities. Some spiral galaxy rotation curves are rising and others are falling at the optical radius, thereby weakening the "disk-halo conspiracy." Observational and theoretical techniques to probe the dark halos of ellipticals are becoming more refined, and various methods are being employed to investigate the shapes of halos. There are useful constraints on the extent of the dark matter around our own Galaxy, whereas in some other galaxies the edge of the halo may have been detected. Microlensing experiments have recently commenced which may soon uncover the nature of the Galactic dark matter. These and the other issues reviewed here have far-reaching implications for galaxy formation and evolution, and a variety of cosmological questions.