If black holes capture everything that reaches their event horizons, what keeps them from devouring the univer

Black holes have a very powerful gravitational pull. However, gravitational pull decreases dramatically with distance: every time you double your distance, the force is divided by four. This means that things in a different solar systems aren't affected that much by a black hole; they're simply too far away.

The emptyness of the Universe.
They are so small that they can encounter very little of its mass.