At one point, these intergalactic vacuum cleaners were actually supermassive stars. When one dies, it generally blows off its gaseous outer layers and the core collapses into an extremely small and dense sphere. Imagine, for example, trying to pick up a tennis ball containing the entire mass of the Sun. The immediate effect of this astronomically high density would be an insanely strong gravitational field. To break free from any gravitational field, you have to be traveling faster than something known as escape velocity. On Earth, spacecraft reach a speed of about 7 miles per second. On some collapsed stars, though, they would have to reach a speed faster than 186,000 miles per second which is more than the universal speed limit, meaning nothing – not even light – could escape.