Are black holes expanding? We know that it is a commonly heard claim that in every black hole there is a universe. It is also believed that the universe is expanding. If we use both of these, can we conclude that every black hole is expanding?
Most$^\dagger$ black holes are indeed expanding, but not because the Universe is expanding. Rather, their size (more precisely their Schwarzschild radius) increases proportionally to their mass, so they grow as they accrete more matter.

It is a common misconception that everything expands along with the expanding Universe. It doesn't. On small scales, i.e. black holes, planets, stars, galaxies, and even galaxy groups, gravity ensures that these things don't increase in size. Only on large scales, i.e. galaxy clusters and beyond, does the expanding Universe pull everything apart.

As for the theory about a universe inside, I… I don't even wanna go there. I'll just refer to HDE 226868's comment.

$^\dagger$Very small black holes will decrease in size due to Hawking evaporation.