What are Black Holes ?
A black hole is a place in space where gravity pulls so much that even light can not get out.
The concept of a black hole was first proposed in 1783, it was Albert Einstein's 1915 theory of general relativity which put the idea on a firm theoretical footing. Einstein showed that gravity can bend the path of light just as it bends the path of any other moving object
The only reason we don't observe this effect in our daily lives is that light moves fast and gravity pulls weak. When this was confirmed by observations, the idea of a black hole became obvious. If you pack enough material together, its gravitational pull should be strong enough to not only bend light's path but also keep it from escaping.
Regular black holes are thought to be formed from heavy stars (perhaps those which start off with masses more than 20 or 25 times that of the Sun
We can't observe black holes directly, but we do see their effect on surrounding material.