It likely is, we just can't detect it on such small scales. It would be a form of matter that interacts pretty much only via gravity, and since gravity is already 40 orders of magnitude weaker than the electromagnetic force we usually use to detect things, it's understandable that it is kinda hard to see.

They might be, but only in very small quantities. The amount of dark matter that we would need to "fix" the galaxy rotation problem... would it affect the orbits of things in the solar system at all? (OK, yes, it would, but observably?) I haven't done the math, but I strongly suspect that the answer is "no".