And they are getting more massive all the time.
Astronomers think most galaxies harbor a black hole at their centers.
The source of this background radiation has been puzzling researchers for more than three decades, and it is now thought to emanate from individual galaxies with active black holes. Focusing on a phenomenon called redshift , which measures the shift in the spectrum of light emitted by a receding object to determine its distance and movement, the scientists looked at the far-off objects, and therefore back in time. They figured out when the black holes had released the energy being measured today, then created a historical timeline of black hole activity. Barger said 10 percent of black holes are active at any time, implying that black hole growth is a slow, ongoing process that can take billions of years. This is much longer than would be expected if the black holes were formed in violent galaxy mergers , as some researchers have speculated.