Over at Astronomy Picture of the Day, they have the time-lapse movie below along with an explanation written by a professional astronomer (it's a bit technical, so we'll try to explain some things further). The movie shows stars in the central part of our galaxy. These stars have moved a lot in the past eight years. Stars don't usually move so fast, but these do.

If these stars are being moved and pulled that fast, then there must be something there that is massive (has a lot of gravity) but compact. This image shows only a small portion of the central part of our galaxy. In other words, there's something that's over one million times the mass of our Sun squished into a region less than one-fifth of a light year. That's really strong evidence for a black hole.

These stars are near a strange object that emits unique radiation. That object, called Sagittarius A* or Sgr A*, is marked with a yellow cross in the time-lapse movie below.

The radiation is consistent with theories about the energy that is released by matter when it falls at very fast speeds as it gets torn apart by a black hole. The subatomic particles get pulled at such high speeds - near the speed of light - that they scream out radio waves.

A rival theory is that there is no black hole. It could just be a very dense cluster of millions of stars. As we take better and better pictures of the center of our galaxy, it's up to professional astronomers to argue if there's still room in the picture for millions of stars.