Here are three unsettling facts: Every galaxy has a black hole at its center; the black hole is spinning; and the black hole's spin is speeding up.
The dense centers of galaxies usually harbor enough matter, pushed densely enough together, to create black holes. The black holes at the center of galaxies are often gigantic, having between a million and a billion times the mass of Earth's Sun. These giants are not standing quietly at the center of their galaxies. They're spinning. And it looks like they's spinning faster now than they ever have before.
Although black holes are exactly what they sound like, giant blots of blackness against the starry background, their edges are often bright. Just as matter circles a star, a disk of matter, called an accretion disk, forms around a black hole. The different pieces of matter in the disk, under intense force from the black hole and the disk itself, heat up and emit visible energy. The motion of this matter can give clues to the motion of the black hole. Scientists can also gauge the motion of the black hole by the jets of matter that erupt from the center of it. They spurt out from either side, perpendicular to the disk. It isn't certain what forces cause these jets, but many scientists believe that the spin of the black hole plays a part.
Scientists have analyzed the way jets stream from these black holes, and the way material is gathered on the accretion disks, and come up with a theory. It seems that long ago, when the universe was only half the size it is now, black holes had a low rate of spin. Some of them remain the same now. The rest, however, have sped up considerably. On average, the spin of black holes has increased over the life of the universe. What makes the difference between a low-spin and high-spin black hole? Another black hole, of course. Simply gulping down regular matter doesn't speed up a black hole any appreciable degree. It needs another heavy-weight. When two black holes merge together, the massive gravity of each causes the resulting larger black hole to spin up like top.