Black holes are compact massive objects from which light cannot escape. The largest of these are the supermassive black holes, which are millions to billions of times more massive than the Sun. Dormant supermassive black holes reside at the centers of nearly all present-day galaxies. Their presence has been detected through the orbital motions of stars and gas near the galactic centers. Locally the mass of a galaxy's supermassive black hole is known to be tightly related to the mass of its host galaxy. Given the very limited extent of a supermassive black hole's gravitational influence, this relation hints at a deep connection between the processes that formed the stars in the host galaxy and the processes that formed the central supermassive black hole. Astronomers would like to follow this connection back in time in order to understand the growth of supermassive black holes in galaxies, but this requires obtaining a census of all the accreting supermassive black holes in the universe.