From what I've read, given the speed of stars rotating within a galaxy and the known mass, the stars' speed is too fast to stay in orbit. Dark matter was proposed to explain this but is it possible that there are undiscovered smaller black holes with no orbiting stars?
MACHOs (massive compact halo objects) are thought to make up a portion of the dark mass and portion of these are thought to be black holes, but there are several very solid reasons why black holes not generally seen as a serious alternative explanation to non-baryonic dark matter:
1) Observed elemental abundances are not consistent with the amount of baryonic matter being produced in the early Universe to explain the dark mass
2) Microlensing searches for MACHOs of the size of black holes formed by stellar evolution rules them out as making a significant amount of dark matter in the Milky Way.
3) Black holes are observable through accretion. For black holes to make a significant amount of dark matter they would have to be in isolated locations within the galaxy so as not to be visible. However most dark matter must be distributed evenly throughout the Milk Way to explain the rotation curve.