No, not controversial at all. Dark matter is one prevailing theory that explains a number of odd phenomenon and observations that standard cosmology gets wrong. Things like the spin rate of galaxies is an example. There may be other explanations that fix this problem (say perhaps super gravity) but none have been any more confirmed that the others.

Dark matter is also unknown in the sense that other than prescription of what it should be, we don't what it actually is. None of our current particles fit the bill and the theoretical particles we've thought could work haven't shown themselves in any tests to make them.