Supermassive black holes underwent periods of exponential growth during which we seem them as quasars in the distant Universe. The summed emission from these quasars generates the cosmic X-ray background, the spectrum of which has been used to argue that most black-hole growth is obscured [1−2] .There are clear examples of obscured black-hole growth in the form of 'type-2' quasars [3−5] , but their numbers are fewer than expected from modelling of the X-ray background. Here we report on the direct detection of a population of distant type-2 quasars which is at least comparable in size to the well-known unobscured type-1 population. We selected objects that have mid-infrared and radio emissions characteristic of quasars, but which are faint at near-infrared and optical wavelengths. This population is responsible for most of the black hole growth in the young Universe and, throughout cosmic history, black-hole growth occurs in the dusty, gas-rich centres of active galaxies.The population of unobscured type-1 quasars is now well understood out to redshift z ∼ 2 by virtue of optical surveys [6] which provide direct measurements around the "break" in the luminosity function where the quasar population outputs most of its luminosity density.Current understanding of the obscured type-2 quasar population is far less complete.Examples of type-2 quasars have, for many years, been confined mainly to radio-selected samples. Radio emission is unaffected by dust and, at low frequencies, is not strongly dependent on viewing angle so the simplest interpretation of optical spectroscopic follow-up of