This happens after the nuclear fuel, which makes the star shine for millions of years, is exhausted. The existence of such black holes has been inferred in cases where the black hole pulls gas of a companion star that orbits around it. Super-massive black holes are found in the centers of galaxies that contain billions of stars. They may exist in most galaxies, and probably formed at the same time as galaxies themselves. They are millions or billions times as heavy as our Sun, as determined from the motions of stars and gas surrounding them.
Material is ejected in jets that emit radio waves, and heated gas produces X-ray emission.
Observations of such X-rays may soon provide insight into the spin of black holes. There are enough black holes in the Universe that there should occasionally be collisions between them. Such violent events send ripples through the space-time fabric of the Universe. Steven Hawking showed in 1974 that every black hole spontaneously and continuously loses a tiny fraction of its mass due to radiation. However, this Hawking radiation is negligible for the known black holes in the Universe, and will not be detectable in the foreseeable future.