The authors consider the idea that galaxy halos are composed of massive black holes as a possible resolution of two problems: the composition of dark halos, and the heating of stellar disks. It is found that in order to account for the amount of disk heating that is observed in our Galaxy and several others, the black holes must have masses MH ≡ 106M_sun;. This heating mechanism makes predictions for the dependence of the velocity dispersion on time, and for the shape of the velocity ellipsoid, that are in good agreement with observations. The authors consider the constraints on this picture set by dynamical friction causing black holes to spiral to the galactic center, by the possible presence of dark matter in dwarf spheroidal galaxies, and by the accretion of interstellar gas by the black holes producing luminous objects in the Galaxy