Dark matter is a significant component of many Local Group galaxies. Spiral galaxies exhibit H I rotation curves that become approximately flat at large radii and that extend 2-3 times beyond the optically visible galaxy. Global mass-to-light ratios (M/L) inferred from rotation curves of spirals are typically 10 M / L for the visible regions (~ 1 - 3 M / L in disks, ~ 10 - 20 M / L in bulges), while the dark matter in halos seems to significantly exceed these values (Longair 1998). This motivates efforts to determine the nature of the dark matter through microlensing in the Galactic halo and toward the Galactic bulge as detailed elsewhere in this volume, and through pixel microlensing of stars in the disk of M31 by dark massive objects in M31's halo (Crotts 1992).
In gas-rich dwarfs the presence of dark matter is inferred as well from H I rotation curves. Some of the less massive dIrrs are rotationally supported only in their centers, while the majority of dSphs studied so far does not show evidence for rotation at all. Chaotic gas motions dominate in low-mass dIrrs, and the H I column density distribution is poorly correlated with the stellar distribution (Lo, Sargent, & Young 1993). In the dE NGC 205, which is tidally interacting with M31, integrated light measurements revealed that the stellar component is essentially non-rotating though the H I shows significant angular momentum (Welch, Sage, & Mitchell 1998). In gas-deficient dSphs kinematic information is based entirely on stars. Most dSphs show no rotation. Their velocity dispersions are typically 7 km s-1. Assuming virial equilibrium velocity dispersions and rotation curves can be translated into virial masses. The derived total M/L ratios of Local Group dwarf galaxies present an inhomogeneous picture ranging from ~ 1 to ~ 80 (see compilation by Mateo 1998).
Compact high-velocity clouds (CHVCs) are a subset of high-velocity H I clouds with angular sizes of only about 1 degree on the sky. They show infall motion with respect to the barycenter of the Local Group. Preliminary estimates place them at distances of 0.5 to 1 Mpc in contrast to the extended nearby high-velocity-cloud complexes (Braun & Burton 1999). Their rotation curves imply high dark-to-H I ratios of 10-50 if distances of 0.7 Mpc are assumed, and masses of 107 M (Braun & Burton 2000). CHVCs may be a significant source of dark matter and may represent pure H I/dark-matter halos prior to star formation. We are currently carrying out an optical wide-field survey to establish whether they also contain a low-luminosity, low-density stellar component, which would imply the discovery of a new, very dark type of galaxy, help to refine CHVC distances and allow detailed studies of their stellar populations.