Transcript: In general, light in the universe is a poor tracer of mass. Galaxies are bright markers of space, however, they poorly represent the distribution of dark matter. The different distribution of galaxies with respect to dark matter is called bias, and it’s a fundamental part of any theory of galaxy formation to explain how and why galaxies cluster the way they do and how it relates to the dark matter distribution. Even if galaxies do not trace the distribution of dark matter, they must move in response to the gravity of dark matter. Those motions can be measured by astronomers with spectrographs, and this allows astronomers to infer the dark matter distribution from galaxy surveys. In addition to the cosmic expansion that affects all galaxies, they have departures from Hubble flow which are called peculiar velocities imprinted by the gravitational interaction with dark matter acting over billions of years. Careful analyses of large surveys of thousands of galaxies enables a fairly accurate measurement of the cosmic density of dark matter leading to the conclusion that it’s about one-third of the critical density.