Data from NASA's Swift satellite has confirmed the existence of active galaxies powered by massive black holes, solving the mystery of the cosmic X-ray background.

If you could see X-rays, the whole sky would glow. Black holes produce massive amounts of X-ray radiation as mass falls into them. But when astronomers counted up the number of active galaxies they could see, there were too few to account for the radiation.

Scientists suspects that the active galaxies - and the black holes that power them - were just obscured by dust and gas that surrounds them. This is especially true when a galaxy faces out own edge-on rather than face-on, because the lanes of gas and dust cluster at the edges. On top of that, the material that dims the X-rays also does the same thing to other wavelengths, so nobody could be sure they had seen all the galaxies there were in the cosmic neighborhood.

The Swift satellite has changed that. Launched in 2004, the satellite carries instruments designed to look into the ultraviolet and X-ray wavelengths, which never reach ground-based telescopes. Some of the X-ray wavelengths penetrate the obscuring material that hides their sources. The satellite has since mapped the entire sky in hard X-rays.

The satellite has mapped active galaxies up to 650 million light years away. From that group, the researchers eliminated sources near the plane of our own galaxy, and the active galaxies that had energetic particle jets. That left 199 in the pool.

Looking closely at that group, the scientists found nine that were obscured by the gas and dust. Knowing that the Swift sky survey undercounts them, the astronomers were able to figure out that such 'hidden' galaxies are about 20 to 30 percent of the total number.

If that proportion stays constant at greater distances, then that would account for the number of massive black holes necessary to create the cosmic X-ray background. That is because the models call for many supermassive black holes to be active about 7 billion years ago, and as one looks at objects further away one is also looking further back in time.