Yes but for it to cause the effect we see with galaxy rotation, there would have to be an absolute shit-ton of it; way more than any theory predicted.

Every type of matter that is floating around out there gets incorporated into stars. The reason for this is that there’s nothing special about the process that makes stars. It’s just a gravitational collapse. It can’t select for one type of matter and exclude another. So for example, if carbon atoms are floating around, they end up in stars. Make sense?

We can use spectrographs to get the proportion of different types of matter in stars, and (with some adjustments for the age of stars) the proportion that should be in the rest of the galaxy.

But then, based on the anomalous spin, there must be a lot more - a lot more.

Or, theories of gravity are wrong, or maybe the speed of light isn’t constant. There are several possibilities, but “missing matter we can’t see” is definitely the simplest. It’s just that, after all these years we haven’t found it.