Black holes form when stars die and collapse in on themselves. They’re a place where the gravity pulls so much that even light can’t make it’s way out. The gravity within black holes is so strong because matter has been compressed into a tiny space.
“They [black holes or massive stars] are typically more massive than the other individual objects around them, and the cumulative effect of gravitational interactions between objects over the long life of a galaxy is that more massive objects tend to end up near the center of the system,” astronomer Brooke Simmons told Mashable.
“That includes black holes, so having lots of black holes at the center of a galaxy is a fundamental consequence of gravity in these large systems made up of billions of individual objects with a variety of different masses,” Simmons said.
With light not being able to escape, black holes cannot be seen by people – making them essentially invisible. Scientists use special space telescopes, like the Chandra X-ray Observatory space telescope, to collect data about black holes and the stars around them.