  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, groups, and clusters of
galaxies to suggest that most of the dark matter may reside in very large halos
around galaxies, typically extending to ~200 kpc for bright galaxies. We show
that the mass-to-light ratio of galaxy systems does not increase significantly
with linear scale beyond the very large halos suggested for individual
galaxies. Rather, the total mass of large scale systems such as groups and rich
clusters of galaxies, even superclusters, can on average be accounted for by
the total mass of their member galaxies, including their large halos (which may
be stripped-off in the dense cluster environment but still remain in the
clusters) plus the mass of the hot intracluster gas. This conclusion also
suggests that we may live in a low-density universe with Omega ~ 0.2 - 0.3.
