Tracing mass and light in the Universe: where is the dark matter?

How is mass distributed in the Universe? How does it compare with the distribution of light and stars? We address these questions by examining the distribution of mass, determined from weak lensing observations, and starlight, around >105 Sloan Digital Sky Survey MaxBCG groups and clusters as a function of environment and scale, from deep inside clusters to large cosmic scales of 22 h −1 Mpc. The observed cumulative mass-to-light profile, M/L (< r), rises on small scales, reflecting the increasing M/L of the central bright galaxy of the cluster, then flattens to a nearly constant ratio on scales above ∼300 h −1 kpc, where light follows mass on all scales and in all environments. A trend of slightly decreasing M/L (r) with scale is shown to be consistent with the varying stellar population following the morphology-density relation. This suggests that stars trace mass remarkably well even though they represent only a few per cent of the total mass. We determine the stellar mass fraction and find it to be nearly constant on all scales above ∼300 h −1 kpc, with M */M tot 0.01 ± 0.004. We further suggest that most of the dark matter in the Universe is located in the large haloes of individual galaxies (∼300 kpc for L* galaxies); we show that the entire M/L (r) profile - from groups and clusters to large-scale structure - can be accounted for by the aggregate masses of the individual galaxies (whose haloes may be stripped off but still remain in the clusters), plus gas. We use the observed M/L ratio on large scales to determine the mass density of the Universe: