I did ponder that for some time.

Others have posted about how the growth of black holes implies matter doesn't "slip off to elsewhere" from them, so I'll let that topic rest. I was wondering about your knowledge at the largest scale of the universe. Have you heard of the Great Attractor? The Vela Cluster? If not, read up on those and see if you might suspect, as I do, that somewhere out there, the Big Crunch is already underway.

I have had that same thought. All that matter has to go somewhere. Of course l an old drummer, so what do l know.

Also called the White Hole Hypothesis. Basically the opposite of a black hole, a hole raditing an abundance of energy in an alternate universe or as a sort of work hole. So far no evidence to support it. Probably won't be in our life times.

If the matter is expelled to another universe the black hole would lose mass to the other universe as well hence losing mass in this universe which is not observed. When matter is swallowed in a black hole its mass increases. Also a black hole is no more a vacuum than say a sun of equal mass for example, if our sun suddenly collapsed into itself as a black hole the orbits of our solar system would not change since the mass of the black hole would be equal to the sun.

Don't see any validity in the theory.

They accumulate more and more mass. They don't lead to anything. Probably.

Black holes, as far as we know do not expel anything, apart from Hawking radiation, . What does universe vacuums mean ?

It's an idea that's been considered, but as of yet there is no evidence to support it.