Black holes are places where ordinary gravity has become so extreme that it overwhelms all other forces in the Universe. Once inside, nothing can escape a black hole's gravity — not even light.
Yet we know that black holes exist. We know how they are born, where they occur, and why they exist in different sizes. We even know what would happen if you fell into one. Our discoveries have revealed one of the strangest objects in the Universe, and there's still much we don't know.
The nearest black hole is many lightyears away, so we don't have to worry about threats to the Earth. This is as close as you'll ever get to one. So come explore!
I took this from HubbleSite, enjoy!