Q&A: Black Holes

Q:
I read on the CNN site that Chandra's instruments had taken a look at the "supermassive black hole at the center of the Milky Way galaxy." I thought that a black hole at the center of our galaxy was still a theory, when was it established as fact?

A:
The evidence for a black hole in the center of our galaxy is very strong. Stars and gas clouds are observed to be moving rapidly around a dark object that has the mass equivalent to about 3 million suns. The only known object that could have so much mass and still be dark is a black hole. So, unless Einstein's theory of gravity breaks down, our galactic center must contain a black hole.