What would we theoretically find if we were able to enter a black hole?
Unlikely that we will ever know for sure.
Any attempt to see inside a black hole would be very difficult as the gravity attraction is so intense that no human would survive - even in a super-duper reinforced spaceship. We couldn't even design a probe that would withstand the enormous gravitation pressure in a black hole - after all, they can swallow stars whole!
It might be possible to infer what the inner workings of a black hole might look like and many cosmologists are working on just that.