What are theories about matter that goes into a black hole?
There are quite a few theories about what happens to matter that is taken in by the black hole.
The first theory is that the matter taken in by the black hole has been transferred to another part of the Universe or, get this, to ANOTHER UNIVERSE.
The second and probably the most obvious theory is that the matter will forever reside inside the black hole and will never be seen again.
The third and my favorite theory is that the matter taken in by the black hole actually explodes out into the Universe, probably as a supernova, when a black hole is near the end stages of its life (supermassive black holes can live for 10^100 years)This theory is also a model for how the Big Bang happened.