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 $>10^5$ SDSS 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 $\sim 300 h^{-1}$ 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 percent of the total mass. We determine the stellar mass fraction and
find it to be nearly constant on all scales above $\sim 300 h^{-1}$ kpc, with
$M_{*}/M_{tot} \simeq 1.0\pm0.4\%$. We further suggest that most of the dark
matter in the Universe is located in the large halos of individual galaxies
($\sim 300$ 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 halos
may be stripped off but still remain in the clusters), plus gas. We use the
observed mass-to-light ratio on large scales to determine the mass density of
the Universe: $\Omega_{m} = 0.24 \pm 0.02 \times b_{M/L}^{2} = 0.26 \pm 0.02.$
