By Ron Cowen
Astronomers have new evidence that a majority of the biggest black holes in the universe lie hidden behind thick veils of dust.
Monster black holes, which reside at the centers of galaxies, betray their presence when their immense gravity induces surrounding matter to generate a brilliant beacon of light known as a quasar. Models of the quasar light suggest that these beacons make a large contribution to the overall X-ray glow of the sky. However, the number of individual quasars actually observed in X-ray and visible-light studies falls far short of the expected population.