It is just a band-aid.

We noticed that some equations only make sense if mass is greater than what we'd expect from what we can see. So for now we pretend that there's this dark matter that we can't see, and with it all math so far checks out.

But we don't know if that missing mass exists at all, or if some formulas are wrong. So it's definitely a "hack", but it works for now and we don't have anything better. And we do expect to find an explanation at some point.