To be more technically correct. "Dark matter" is the word we use to describe the thing that would explain why galaxies and the stars within them do not move in

To be more technically correct. "Dark matter" is the word we use to describe the thing that would explain why galaxies and the stars within them do not move in accordance with the gravitational model we believe explains what we see. We assume that our gravitational model is correct and that an additional item is needed to bring the model into agreement with the observations made. When a model seems to work so well, we are generally unable to go back and look at the basis (and assumptions) on which our theories are made. This applies in all areas of our lives, including all of our science. That is the nature of who people are.