Well, you are half right.

Something has been observed through telescopes of various kinds. Part of the thing that they observe is the absence of light and other radiation at certain points in the skies. This means that there is something that is there that astronomers and cosmologists are trying to explain. This is the half that is not imagination.

The imagination part is the Black Hole Theory.

There are 3 major Big Bang Theories that don't fit together. There are 4 major Black Hole Theories that are all different. None of the Black Holes produced by the major Black Hole Theories could exist in any of the universes produced by the major Big Bang Theories.

The reality is that something exists out there that we have named "Black Holes." We have used our imagination to try to guess what they are.