Space is full of objects that push the boundaries of imagination, but few do it quite as effectively as a black hole. At its simplest, a black hole is a region of space where gravity has become so overwhelmingly powerful that nothing, no matter, no light, nothing can escape its grip. They form when massive stars reach the end of their lives and collapse catastrophically inward, crushing an enormous amount of mass into an extraordinarily small space. The result is an object so dense that it warps the very fabric of space and time around it.