One of the major problems faced by science today is that our current model of physics is woefully incomplete, most infamously so in regards to the inability of physicists to make the theory of relativity and quantum theory work together. This problem becomes extremely apparent when it comes to figuring out how black holes work: relativity tells us that nothing would be able to escape a black hole, causing a fundamental loss of the quantum information that that made up the material that falls into it, but quantum mechanics tells us that that information is indestructible, meaning that that loss can’t happen to begin with.
Needless to say, a large number of ideas have been put forward to explain the black hole information paradox. Amongst these theories, there are two prominent ideas: one is that black holes are actually 2-dimensional objects that project the illusion of being 3-dimensional structures in our universe, and the other is that they’re portals to another universe altogether.
The idea that black holes might be 2-dimensional holograms was first proposed by physicist Leonard Susskind in 1995, based on an earlier idea from Stephen Hawking that a black hole’s internal entropy should be proportional to its area, rather than its volume, as one would expect from a 2D hologram. Since then, theoretical models using the holographic principle hypothesis have met with better success in reconciling the models of classical physics and quantum mechanics. This idea can also be applied to the rest of the universe as well, but its effect is most pronounced when it comes to explaining black holes.
An alternative solution, proposed by Stephen Hawking, is that the quantum information is neither stored nor destroyed, but rather that it winds up in another universe altogether when it passes through a black hole. This way, black holes can draw in as much material as one can imagine, but since the information is preserved (somewhere), it still jives with quantum theory. However, he does caution that this would be a one-way trip, since nothing should be able to escape from a black hole, and that the idea only applies to elementary particles, since larger objects, such as a spacecraft, would be unlikely to survive the gravitational stresses involved.
In addition to this, Hawking also notes that if matter from our universe is exiting into other universes, that means that the inverse might very well be true, and particles from other universes might be streaming into ours.