Black holes don’t emit light, but they still shine. 

They do so because of accretion disks, but those disks don’t appear around black holes of all sizes. There could be incredibly huge black holes out in the universe that we can’t see, because they’ve gone really dark.

Astronomers have seen some massive black holes. They’ve seen black holes billions of times more massive than our sun. What they haven’t seen is black holes much bigger than 50 billion solar masses. While no one’s complaining about that, a few are asking why, and one guy has a guess. At the University of Leicester, Professor Andrew King took a look at the disks of gas and dust that surround black holes. These orbit the black hole a bit like water circling a drain—in that there’s a decent chance that matter caught up in that orbit is going to fall into the black hole, feeding the thing. In the meantime, the disk heats and shines, letting us “see” the black hole it is circling.

As the black hole grows, the disk doesn’t get bigger. The opposite tends to happen. Matter breaks up and away, often forming new stars. The bigger the black hole, the less capable it is of keeping a disk around it. The upper limit for a black hole that can keep any sort of disk is about 50 billion solar masses. Since the disk is what feeds the black hole, this becomes a soft limit for black hole size in general. 

Not a hard limit, though. If two black holes collided, they could make a black hole with a mass of up to 100 billion solar masses. (Also, that would be incredibly cool.) Because it had no disk of hot gas around it, we would have trouble seeing it. For those who are now looking over their shoulders, it wouldn’t be completely invisible. It would still shape the motion of nearby objects and distort the path of light, so we probably would notice one if it were perched just on the edge of the solar system. (Although if we saw it, what would we do about it?) Still, it would be much, much harder to spot than other, smaller black holes. The bigger they get, the more impossible it is to see them.

[Source: How Big Can a Black Hole Grow?]