Most of the mass in galaxies like our own Milky Way does not reside in the stars and gas that can be directly observed with telescopes. Rather, around 90 percent of the mass in a typical galaxy is "dark matter," a substance that has so far evaded direct detection. How can scientists make this astonishing claim? Because, although we have yet to detect dark matter, we can infer its existence from the gravitational pull it exerts on the luminous material we can see. Another facet of the dark matter problem comes at larger scales, where the total amount of mass in the universe exceeds the inventory of atoms we think were made in the Big Bang. A third indication of something missing comes from the evolution of the large-scale structure in the Universe, where fluctuations in the dark matter density are needed to seed the formation of the tendrils and filaments of galaxies we see in observations. So what is dark matter, and how might we find out? Determining the nature and distribution of dark matter is one of the most pressing (and most interesting!) open questions in modern science—it resides at the interface of particle physics, astrophysics, and gravity. Many candidates for dark matter have been suggested, from the ghostly axion (particles with a tiny amount of mass) to Weakly Interacting Massive Particles (WIMPs) that weigh in at 100 times the proton's mass. In this unit, we shall review the observational and theoretical evidence for dark matter, and describe the attempts that are under way to find it.