What if they are just extremely dense objects with extreme gravity. All matter that goes into a black hole has to go somewhere. I think that’s pretty much what they are. I remember reading once that if the earth got sucked into a black hole it would have the same mass but be the size of a pea, which boggles the mind to think about. Either that or the mass is converted to energy…. it has to be one of those 2 things, I really don’t know for sure though.
I’m not going to touch black holes with a 10 megaparsec pole. They are, I believe, an artifact of a partial theory of gravitation, and someday, will take their proper place in the history of science, alongside phlogiston and the luminiferous æther.
A better question to ask is: where does all the information go, that necessarily must disappear when a black hole forms? This one (the Black Hole Information Paradox) arises from the fact that the entropy of a black hole has been shown to be proportional to the surface area of the event horizon.
Using the equations of Einstein-Maxwell gravitation and electromagnetism (basically, Maxwell theory rewritten into Riemannian space), the very first black hole solutions show that the only (intrinsic) properties a black hole can have are: mass, angular momentum, and charge. All other information about what fell into the black hole disappears. This is called the “no hair conjecture”, after a comment by J.A. Wheeler (“a black hole has no hair”, which itself paraphrases a geometrical result in mathematics).
However, that violates the second law of thermodynamics, quite badly so. If black holes had no entropy, one can violate the second law simply by throwing matter into them (and that is a second law violation in the rest of the universe, not within the black hole itself). Thus, it was conjectured (Jacob Bekelstein and Stephen Hawking) that black holes had a fourth property, entropy, and it was quickly found to be proportional to the event horizon surface area — which came as quite a shock to everyone, because that area does not increase fast enough as matter crosses the horizon. The second law of thermodynamics was still violated.
Now, the second law of thermodynamics is very well tested, and has worked well everywhere else, so we are pretty confident about that; meanwhile, black hole theory is really just entering its adolescent phase, so our understanding there is still far from complete.
The favourite resolution, in fact the only one which seems at all reasonable, is found in Hawking radiation (see note): extreme quantum fluctuations just above the event horizon produce particle-antiparticle pairs, one of which will fall into the black hole, while the other escapes. For complicated reasons of energy conservation, the particle is the one which escapes.
This alone is not sufficient to resolve the paradox, and it takes some very wicked mathematics to go any further. In 1997, Hawking, along with Kip Thorne, had made a bet that entropy was lost in black holes, much to the consternation of pretty much everyone else. In 2004, using one of those wicked mathematical models, Hawking finally conceded defeat, though Kip Thorne is not convinced, and steadfastly refuses to pay up.
Personally, I believe the situation with black holes is similar to that of the paradoxes of classical physics in the late 19th century, which were even as early as 1870 beginning to put the nails in the coffin of Newtonian physics. By 1905, the process was pretty much complete. The situation with General Relativity is, I believe, pretty much the same, though it has taken us less than 100 years to get there (the centenary is still 3 years away, almost to the day).
I do not suggest tossing out General Relativity and starting afresh; that would be insane, as that theory has far too many successes, even if some discrepancies are starting to show up (planetary precession, for example), while the failures are immense (above). Just as special relativity reduces to Newtonian mechanics in the limit of very low speeds (compared to the speed of light), any successor to General Relativity must reduce to that theory in some appropriate limit.