Giant collision created galaxies devoid of dark matter

Sure, fine. But we're seeing two modes of galaxy rotational physics. One mode, uncommon, that matches classical predictions. A second, more common, which matches only if we add a distribution of unseen matter. That's compelling.

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.