Is There a Black Hole at the Center of Our Galaxy?

To identify a black hole, you need to see its gravitational effect on stuff around it. And so the argument that something is a black hole is that you measure its mass by looking at how hard it pulls on things gravitationally near by. Not everything with gravity has a black hole r if the earth has gravity, you can measure its pull on the moon. But if you can measure something's mass and its radius, and the radius is really, really small, that means that the object is really dense. It's very compact. below a certain radius, an object of that mass has to be a black hole,. as far as we know.