A recently published article claims that new calculations reveal that black holes probably do not have an infinitely dense zero radius singularity inside, but rather wormholes to somewhere else. (The article just says “another region of space”.) My question is that if black holes have an exit, where are they? We’ve never observed one as far as I know. Even if the"white holes" are in another universe, it would still have to be asked why we don’t have any in our universe coming from somewhere else.

Purely theoretically the other exterior region is a copy of the BH exterior region: a parallel universe, if you like. However if such white holes were real it is not inconceivable for both the white hole and the black hole to share an exterior region

BTW this different from the standard white-whole-featuring extended Schwarzschild solution, which features a wormhole called an Einstein-Rosen bridge, which is purely an artifact of a particular spatial-slicing and is most definitely not traversable.

One problem with such white hole-containing metrics is that they are not stable with respect to global hyperbolicity. In layman’s term this means really, if such solutions were physical, there’s no rhyme nor reason as to the connections between various different exterior regions.

I imagine the answer is ‘no’ or ‘impossible to say’, but could it be that the ‘other side’ of a black hole is actually an entire universe (i.e. there is just one of these in our universe - and it *is* the universe)

If you would like to learn more about black holes/white holes/wormholes and see via animations how they would realistically look and interact with spacetime (from our best mathematical models of events); then check this website. It was made by professor, Andrew Hamilton of the University of Colorado, who has been publishing papers on the subject since the 1980‘s till today, while working in tandem with the National Science Foundation.

http://jila.colorado.edu/~ajsh/insidebh/intro.html

"What is beyond the Antihorizon of a black hole? In a real black hole, the other side of the Antihorizon, or illusory horizon, is the star that collapsed, redshifted to blackness.

However, the Schwarzschild geometry has a simple mathematical form, and that form can be extended analytically. The mathematical extension consists of a second copy of the Schwarzschild geometry, reversed in time, glued along the Antihorizon. The complete analytic extension of the Schwarzschild geometry contains not only a Universe and a Black Hole, but also a Parallel Universe and a White Hole. This is simply a mathematical construction, with no basis in reality. Still, it is cute that even the simplest kind of black hole, a Schwarzschild black hole, harbors alien mathematical passageways. “

http://jila.colorado.edu/~ajsh/insidebh/penrose.html