CDM is needed for a bunch of reasons, this is an incomplete list:
1) to get the gravity right in spiral galaxies, as Kaptain K already pointed out
2) to get the gravity right in clusters of galaxies, as inferred from their motions
3) to get enough gravity for 1 and 2 above without messing up the observed fractions of light nuclei that were formed early in the Big Bang
4) to get a flat universe (in comoving coordinates) as expected from inflation. Even if you don't accept inflation, the fact that it is near flat would be spectacularly suprising if it wasn't virtually exactly flat.
5) to explain how galaxies were able to form from a nearly homogeneous matter distribution (baryonic matter alone could not have caused galaxies to form so quickly).
6) to avoid altering the theory of gravity, a very successful theory with excellent axiomatic underpinnings, with something completely ad hoc that so far has not proved to be promising.
7) to allow the possibility that not all matter interacts with light, which otherwise would be a very photon-ocentric view of the universe just because we have eyes.