Seriously, they aren't. Some physicists make them out to be these incredible things, but really they are just hugely massive gravitational bodies and they act in very predictable ways. The recent detection of Gravitational Waves and what they detected just proves my point.
There is this big question among those who study black holes as to what happens when matter is sucked in. Some say it is destroyed and some say it cannot be destroyed because of the law of conservation of information. Well, both points are ridiculous and pointless. Might as well talk about how many angels can dance on the head of a pin. Why do I say this? Well, the mass. When matter gets sucked into a black hole, the black hole gains mass. Not all matter may get sucked in, some may get converted to energy and some may be flung away before it enters the event horizon.
One scientist was so desperate to prove that information was not lost that they resorted to measuring the surface area of the black hole, noting, much as the known increase in mass, that the size also increases and so the surface area goes up. Sometimes I think these specialists scientists get too focused on the minutiae and forget to let logic and reason in.
Okay, so what proof do I have. Well, the math. Now, I'm not going to put down how all this is calculated, or even what the precise final equations are that can be compared. For one it is hard to do in a blog. Another, either you are a physicist and can look them up or you aren't and they wouldn't make sense to you even if you saw them. Michio Kaku covered it well in his book Hyperspace. But anyway, I digress. When you calculate what a black hole should look like you get a strange result. It doesn't make a lot of sense even if you are an expert. Interestingly enough, you get a very similar result when you do the same calculations for the big bang singularity.
So what does this mean. Well, that depends all on the theory of the Big Bang you follow. I find string theory compelling. The added dimensions make a lot of sense. When you look at it that way, in the moments after the big bang, new dimensions come into being and matter is created from those dimensions. Generally it is described as the vibration of a string, which is why they call it string theory. But the crux of the idea is the the change in the dimensional nature of the universe leads to the creation of matter, first at the quantum level, then at the atomic level. From the nature of the math results, I have concluded that the process inside a black hole is so similar to the conditions during the big bang, just moving in the opposite direction, that the dimensions get crushed. This answers the conservation of information conundrum because it means that a state that started with the Big Bang is being reversed on a small scale. Yes that means matter would cease to exist, but not the dimensions and vibrations that created it in the first place. It is hardly different than the observable states of matter - gas, liquid, and solid and the change between them. It is just a more fundamental change under conditions that are hard for us to comprehend. The temperatures and pressures within a black hole are extreme.
As a result, black holes become quite easy to understand. They are just mini-reverse-big-bangs. Nothing hard to comprehend and nothing that requires a great mind to figure out. Black holes are not gateways to other places, they are just huge chunks of matter that gobble up any matter that gets too close. They convert matter back into its constituent components and grow in mass and in the diameter of their event horizon. Now, as a gravitational body off extreme size there are bound to be some interesting things going that could produce some of the interesting tales SF authors have proposed, but the only thing you'll get by crossing the event horizon is absorbed into the black hole and your matter converted into a higher state.