English for teens. Age 16-19

 There is even a critical distance close to a black hole where the escape velocity is equal to the velocity of light; any object passing beyond the critical distance can never return to the normal universe. No one has hardly ever seen a black hole. The fact that it is black, possibly small and, hopefully, far out in space, makes it almost impossibly undetectable. How then do we know about them? The answer is that they are predicted to exist by the Einstein's general theory of relativity that was still designed to explain gravitational fields, especially the very strongest ones. This theory has so far been good enough at explaining the strange things that astronomers will see in space."