Recall, Newtonian equations seemed to explain gravitation, until we realized they didn't really, only from one inertial perspective. So, something akin to that might be the case here. We figure that the "seemingly missing" force must be from "something" and so infer that this something is Dark Matter/Energy. It could be the case that we simply are not "using the right perspective" or that our method of calculation is just missing something.
Yeah, when I first learned a bit about this dark stuff and then heard about a galaxy with none of it - I was like, well that galaxy looks just fine ( could be it's not ), so this dark stuff ain't necessary - or it could be like you say, since we think something is there we know nothing about, could be it's not there and we're not understanding something that can explain our results/readings.
Actually, it's the opposite. The fact that you can find galaxies that behave as if there's no dark matter strengthens the conclusion that there IS something dark and heavy and at the same time demolishes alternate theories (for how would these galaxies without dark matter behave as they do in that case?).
Forgive me, I'm out of my depth here - you're right, it's their spin that's off that alerts us to a lack of dark matter, not something measured.
