M87 Black Hole 
No one has ever seen a black hole. That makes sense, because black holes are completely black -- nothing escapes from them, including light. Yet astronomers have discovered hundreds of them -- including some monsters that are bigger than our solar system. They use a variety of techniques to find the black holes -- they just don't actually see them.
The biggest and most massive black hole for which astronomers have a good measurement is at the heart of the galaxy M87.
GEBHARDT: The black hole mass we measure is 6.6 billion times the mass of the Sun -- it's the biggest black hole in the nearby universe.
That's Texas astronomer Karl Gebhardt, who led a team that measured the black hole's mass. The team measured the motions of stars not far from the black hole, which are accelerated by the black hole's powerful gravity.
M87's black hole is so large that one day astronomers should be able to take a picture of its event horizon -- what makes the black hole black; anything that crosses the event horizon is gone for good. That would offer the final proof that black holes really exist.
GEBHARDT: It's about three times the size of Pluto's orbit -- just the event horizon alone. And it could swallow our solar system whole if it happened to wander too near to us -- that's not going to happen. But it is the best candidate we have for trying to observe the event horizon. We don't know whether black holes are black holes. To actually determine if an object is a black hole, you need to have some type of proof of an event horizon -- that's the defining property of the black hole. And that doesn't exist yet.
Script by Damond Benningfield, Copyright 2011