Now this is a fun topic.
Okay first thing's first: black holes are a FACT and are accepted as such by physicists and cosmologists alike. In fact, the #1 person to speak to about black holes is none other than cosmologist Steven Hawking.
Now, according to scientific knowledge, a black hole is the result of the death of a super-massive star. It got to the point at which its gravity overpowered the nuclear energy it produced, and all of its mass colapsed towards the center. Black holes give physicists a hadache because the gravitational force is near infinite, and causes the laws that govern physics as we know it to be invalid.
I would imagine the inside of the event horizon of a black hole would be somewhat viewable, as super-small black holes glow. This glowing seems to imply that, while light can't escape the gravitational pull, it isn't destroyed. At the center, you would see a super-dense sphere of "matter" (though, it wouldn't be matter as we conventionally think of it). Some theories even go as far as to suggest that black holes create a bridge between universes, though we are unlikely to ever know the truth.
Based on the cycling of the natural world, I have come to theorize that black holes are much like the universe before the big bang. To be more specific, I think it's a continuing cycle where (nearly) all black holes eventually condense into one super-massive black hole, get to the point where the mass is forced to revert back to energy, and the universe begins anew. Though, I doubt this theory will ever get its chance to be tested.
A white hole isn't really gravity in reverse. In many theories, it is the answer to a black hole: all the matter that gets sucked in gets spewed back out. While this is an interesting concept, I personally don't view it as a plausible object. Why? Well, if everything a black hole took in came out, they wouldn't get bigger. However, smaller black holes can combine to become larger ones, therefore we know the matter is not coming back out. The closest things I know of to the concept of a white hole are pulsars and gamma-ray bursts. In both cases, energy is being spewed out of a stellar body. With pulsars, it's a neutron star emitting beams of EM radiation. Gamma-ray bursts, on the other hand, occur during the collapse of massive stars and can be thrown out by black holes. However, neither case can classify as the defininbg example of the white hole object.
