And almost offtopic, in a scifi story, an advanced civilization found a way to extract energy from these systems billion of years ago (or use them just to store stuff, lot of it). Taking positive energy out of the system creates negative energy which is inflating the universe faster and faster. Most of them don't care because it will take billion of years for matter to be stirred into pieces. Some of them think it's not a good idea and should be controlled somehow but cheap energy is too tempting.