Saying "just dark matter" might be missing what could be a whole category of types of objects and particles. The wiki for dark matter is rather long and detailed. There isn't one type, that's part of the problem of seeing unexplained influences that need certain properties but by principle is hard to detect normally.
In short, something is going on in the observed evidence of what is termed as dark matter. Normal matter we're familiar with doesn't work. So we make models of things it might be and try to verify them.