I would think this is strong evidence that dark matter exists. There have been theories like eg. The law of gravity does not hold at long distances, or some ot

I would think this is strong evidence that dark matter exists. There have been theories like eg. The law of gravity does not hold at long distances, or some other inaccuracy in our math. But if that were true, then we would expect it tobe universal. That some galaxies rotate just as we would expect (with gravity proportional to inverse square distance), and others operate differently indicates that there isn't a fundamental flaw in our laws of nature. Some galaxies just have more dark matter than others.