  We show that the observed rotation curves of spiral galaxies constrain the
sound speed of the dark matter to be $c_s < 10^{-4} c$, where $c$ is the speed
of light in vacuum. Using the Modified Chaplygin Gas as a representative
example of a class of unified dark energy models incorporating an effective
dark matter component with a non-zero sound speed, we determine the most
stringent constraint to date on the value of the constant contribution to the
equation of state parameter in this class of models. Finally, we explain the
reason why previous constraints using the Cosmic Microwave Background and
Baryonic Acoustic Oscillations were not as competitive as the one presented in
this paper and discuss the limitations of the recently proposed Extended
Chaplygin Gas.
