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Q:
If nothing can escape from a black hole, then won't the whole universe eventually be swallowed up?
A:
A basic answer to your question is that the universe is a big place. In
particular, the size of a region where a particular black hole has
significant gravitational influence is quite limited compared to the size
of a galaxy. This applies even to massive black holes like the one found in
the middle of our galaxy. This black hole has probably already "eaten" most
or all of the stars that formed nearby, and stars further out are mostly
safe from being pulled in. Since this black hole already weighs a few
million times the mass of the Sun, there will only be small increases in
its mass if it swallows a few more Sun-like stars. There is no danger of
the Earth (located 26,000 light years away from the Milky Way's black hole)
being pulled in.
Future galaxy collisions will cause black holes to grow in size, for
example by merging of two black holes. But, collisions won't happen
indefinitely (because the universe is big and because it's expanding), and
so it's very unlikely that any sort of black hole runaway effect will
occur.
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