What exactly are black holes?

Black holes are... things that are incredibly dense and they have an extreme gravitational force, so strong that not even light can escape from it. If black holes suck in all the light, that means that we can't see them, right? Wrong. Scientists can study how the immense pressure of the black hole affects nearby stars. Also, they might be able to see them if they find out that stars are orbiting the black hole. A black hole is formed when a massive star dies. The centre of the star collapses and a supernova occurs. After that, a black hole would form. Don't worry, our sun isn't massive enough to turn into a black hole. At the centre of a black hole, all laws of physics are thrown away. Where do all the things it sucks up go? No one knows.
It is proven that at the centre of large galaxies, there will be a supermassive black hole. These black holes can have the mass of 1 million suns. Our supermassive black hole, Sagittarius A, has the mass of 4 million suns!

Artist's concept of a black hole
It is predicted that at the centre of a black hole, time dilation occurs, which means that time slows down. A black hole has a boundary, called the event horizon. If you go in there, you are never coming out.
Black holes have been found to emit x-rays, although no one knows why.