All we have are theories, so here are a few of those theories.
One concept, a part of the vacuum solution to Einstein’s filed equations, is a worm hole that leads to a white hole. A white hole is the theoretical opposite of a black hole. Where a black hole attracts matter, a white hole will spew it our. Since the two are connected by the worn hole, what goes in one comes out of the other. That is a serious oversimplification of the complex works of Einstein, but it gets the basic point across.
Theoretical physicists have extensively pondered the contents of a black hole. Despite the fact that they suck in material that strays too close, they are empty. The mass of a black hole is confined to an infinitely small point at its center, called a singularity. A singularity is surrounded by a vast sphere of darkness called the event horizon. The event horizon and the gravitational pull of the singularity are strong enough to capture light, so there is nothing for astronomers to measure. If it cannot be measured, then it must not exist. So, scientists believe that there is nothing inside of a black hole except the singularity itself.
Einstein and many who have followed him, believed that space and time are folded inside the singularity and that it is even possible that our universe began as a singularity that collapsed, then released all of its matter at a huge rate of speed. That is the explanation why everything in space is expanding away from everything else.