As far as I understand the problem with dark matter is also that there isn't a single dark matter theory, but lots of different ones with a lot of very variable tunables. So the problem is that dark matter by itself isn't that predictive.
The term dark matter itself is kind of misleading in the first place. The math in general relativity just doesn't add up when applied to certain observations. They've essentially gotten 2 + 2 = 5. In order to fix that they just assume that one of those 2's was actually a 3 somehow, and the extra 1 it picked up got labelled as dark matter. In other words, dark matter is just a term for some missing numbers somewhere in the calculation. Based on the different places where the extra numbers might be included they think it's something with mass, but really that's just a guess based on the existing formula and the changes that would be necessary to make it match the observation.
