“The deepest mystery, the singularity of the black hole, is not completely understood and is a mystery – an unknown – at the edge of physics as we know it.
Singularities make black holes look like bottomless wells. The formulas of physics break down when we move towards infinity. The exact same happens when you count backward through the beginning of the universe. Through the beginning of the Big Bang.
This coincidence has made some physicist speculate if Big Bang could be a backward black hole – A white one – That white holes inhabit the backside of black holes. In that way they could be a portal out and into other universes.
I once saw a lecture by the Italian astrophysicist Carlo Rovelli. He has a theory, that black holes, only from our perspective, only from our timeline, are black.
Imagine that everything in them falls into this eternity, and while they are falling down time moves extremely slow when seen from the outside. At a point the fall, the collapse, will reach a quantomechanic boundary, where both spacetime and gravity are quantified. Here it hits a sort of wall, and then it leaps upward. The black hole then turns and explodes outward and thus become a white hole. From the perspective of the matter, that is on its way down it might only take a few seconds but seen from the outside it will take an extreme amount of time.”