We think so. If you study the motion of the stars and gas near a region called Sagittarius A* ( pronounced 'A star') you notice that it is moving faster than can be accounted for by simply adding up the mass of the gas and stars you see. Various estimates indicate that as much as 500,000 to 1 million times of the mass of the Sun might be in a black hole in the Galactic center. Other astronomers feel less than 100,000 solar masses may be closer to the truth.