|MadSci Network: Astronomy|
Supermassive black holes are found in the very centers (the nuclei) of large galaxies. We don't know of any supermassive black holes that are found elsewhere, so these are all quite far apart. The (centers of) our Milky Way galaxy and the Andromeda spiral galaxy are over 2 million light years apart. In general, black holes are not giant vacuum cleaners that suck in all matter. They will absorb anything that lands on them, but black holes are quite small in diameter and so they can only absorb a small amount of matter at a time. For example, the black hole at the center of the Milky Way has a mass of about 2 million times the mass of the Sun. That sounds like a lot, but it has taken the entire history of the Universe (about 14 billion years) for the black hole to grow to that mass. At this average rate, it takes 7000 years to absorb as much mass as the Sun has. Now galaxies do sometimes merge, and the black holes at their centers probably merge also, but the rate of mergers is also low. So it's not the case that all the supermassive black holes will merge together into one big one that contains all the matter in the Universe.
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