Contribution of quasar-driven outflows to the extragalactic gamma-ray background

The origin of the extragalactic gamma-ray background permeating throughout the Universe remains a mystery forty years after its discovery(1). The extrapolated population of blazars can account for only half of the background radiation in the energy range of similar to 0.1-10 GeV (refs 2,3). Here we show that quasar-driven outflows generate relativistic protons that produce the missing component of the extragalactic gamma-ray background and naturally match its spectral fingerprint, with a generic break above similar to 1 GeV. The associated gamma-ray sources are too faint to be detected individually, explaining why they had not been identified so far. However, future radio observations may image their shock fronts directly. Our best fit to the Fermi- LAT observations of the extragalactic gamma-ray background spectrum provides constraints on the outflow parameters that agree with observations of these outflows(4-7) and theoretical predictions(8,9). Although our model explains the data, there might be additional contributing sources.