1. Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
2. Spitzer Science Center, California Institute of Technology, MS220-6, 1200 E. California Boulevard, Pasadena, California 91125, USA
3. Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
4. Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Rd, Victoria, British Columbia V9E 2E7, Canada

Correspondence to: Alejo Martínez-Sansigre¹ Correspondence and requests for materials should be addressed to A.M.-S. (Email: a.martinez-sansigre1@physics.oxford.ac.uk).

Supermassive black holes underwent periods of exponential growth during which we see 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, 4, 5}, but their numbers are fewer than expected from modelling of the X-ray background. Here we report 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. We conclude that this population is responsible for most of the black-hole growth in the young Universe and that, throughout cosmic history, black-hole growth occurs in the dusty, gas-rich centres of active galaxies.

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