There is an opinion that in the cores of galaxies can be massive black holes. They are a source of energy, providing the activity of these space objects.
What causes black holes? Their formation may be associated with compression of massive stars at the end of their "life". Also black holes can occur due to huge concentration of substance in the center of a sufficiently massive star systems.
Take some weight substances, which were in relatively small volumes that are critical for a given mass. Under their own gravity this substance starts uncontrollably to shrink. Comes gravitational catastrophe. With the compression of a given mass increases its concentration. There comes a time when the force of gravity on its surface becomes so great that to overcome it would be necessary to develop a speed exceeding the speed of light[?]. But such speeds distribution of physical interactions do not exist. Therefore, the black hole, like a giant funnel that draws in all, nothing blowing out. She does not let go even own light! If this black hole, sucking in itself the surrounding substance, increases its size.