Evidence for Dark Matter We are now going to move on to another of the most important current mysteries in cosmology: dark matter. To set the stage, we will take a census of the mass in the universe, and show that it has a component that we cannot currently identify or even characterize all that well. Throughout this lecture it will be useful to keep in mind the current critical density that would make the universe at by itself. That density is ?crit . 10¡26 kg m¡3, which is about ?crit . 1:4£1011 Mfl Mpc¡3. Known Mass Let's start with the easy stufi flrst: stars. As an especially easy way to estimate the contribution, we note that the number density of galaxies similar to the Milky Way in the local universe is about nMW . 0:003 Mpc¡3. The Milky Way has about 1011 Mfl in stars, so this would lead to a density parameter of ?=?crit = > . 0:002. That's pretty far short of the critical density! Of course, not all galaxies are the size of the Milky Way. Adding together the estimated stellar mass from...