WHERE IS THE DARK-MATTER

How much dark matter is there in the universe and where is it located? These are two of the most fundamental questions in cosmology. We use in this paper optical and X-ray mass determinations of galaxies, group s, and clusters of galaxies to suggest that most of the dark matter ma y reside in very large halos around galaxies, typically extending to s imilar to 200 kpc for bright galaxies. We show that the mass-to-light ratio of galaxy systems does not increase significantly with linear sc ale beyond the very large halos suggested for individual galaxies. Rat her, the total mass of large-scale systems such as groups and rich clu sters of galaxies, even superclusters, can on average be accounted for by the total mass of their member galaxies, including their large hal os (which may be stripped off in the dense cluster environment but sti ll remain in the clusters) plus the mass of the hot intracluster gas. This conclusion also suggests that we may live in a low-density univer se with Ohm similar to 0.2-0.3.