Where is the Dark Matter?  How is it distributed in the Universe?  How does it relate to the underlying distribution of light, stars, and baryons? I will discuss these questions using recent gravitational lensing and other observations that trace the mass distribution from small to large scales and help reveal the connection between the dark and bright side of the Universe.

 

We show that while the observed mass distribution is considerably more extended than light on galactic scales — representing the large dark matter halos around galaxies — this trend changes dramatically on scales larger than a few hundred Kpc, where the mass, light, and stars trace each other remarkably well, indicating the 'edge' of the dark matter distribution on these scales. The results further suggest that, unlike previous expectations, most of the dark matter in the Universe may be located in large halos (~300 Kpc) around galaxies, with no significant increase in the dark matter component on larger scales;  groups, clusters, and large scale structure appear to be made up mostly by the total mass of the individual galaxy members, including their extended halos. We also find that the stellar mass fraction is constant on these large scales, with stars comprising only ~1% of the total mass.

 

How can stars, which represent only ~1% of mass, follow the total mass so well?  Where are the rest of the baryons?  I will discuss the connection between baryons, stars, and mass on these scales, and the implications for cosmology, the mass-density of the universe, the cosmic baryon fraction, and galaxy formation.