Hello Graham, welcome to the PF.
My understanding is that black holes are simply objects where matter has become so dense that not even light can escape from them due to their gravity. I dont see any need for other universes or gateways of some kind. Also most believe that there isnt even a true singularity at their center. It is probably the boundary from which matter or energy can never return which makes them seem mysterious, however they are very much still in our universe.