Among other things, general relativity theory of Albert Einstein predicted the existence of one of the strangest cosmic phenomena - black holes. Recent hypothesis suggests that our galaxy is in the midst of a massive black hole. Why are these holes "black"?
It is known that mass distorts space. We also know that the path of light is affected by gravity. We also know that Einstein's theory of general relativity predicts the existence of one of the strangest objects imagined by physicists - black holes.
What are black holes?
If you could produce sufficient deformation in the structure of space-time by placing a huge mass in a volume small enough, then the space-time continuum would be so strongly curved in that area that nothing, not even light, no could escape. And since nothing can travel faster than light, anything that enters that region of space-time would be trapped inside until various quantum effects would allow it to escape.
Black holes are different sizes and have different masses. It is only necessary that enough matter (mass) is concentrated in a small volume so that it collapsed under the influence of its own gravity.
Stellar black holes form when stars at least 20 times more massive than the Sun exhausts its nuclear fuel and can no longer maintain the reactions that occur within them. They cool quickly collapsed because of its own gravity and shock wave thus generated leads to explosion of the outer covering of the star in a colossal scale cosmic phenomenon that is called "supernova".
Singularity and event horizon
In its center there is what is called a "singularity" of an entire star mass concentrated in a point in space. Around it is an invisible coating known as the "event horizon". Is a limit point cosmic level. Once exceeded this boundary, nothing, not even light can not escape the black hole gravity than through quantum effects and processes.