  Galaxy rotation curves are considered to be convincing evidence for dark
matter or some dynamically equivalent alternative mechanism. Starting only from
the rotation curve data, we present a model independent approach of testing a
general hypothesis that dark matter has the properties of a barotropic fluid.
It is shown how the speed of sound squared can be expressed in terms of
rotation curve data and their radial derivatives and how model independent
constraints can be obtained from the requirements that it is confined between 0
and $c^2$. Using the Milky Way rotation curve data available in the literature,
we obtain the constraints on the barotropic fluid speed of sound and illustrate
the potential of this approach. Technical challenges, limitations and possible
future extensions and improvements of the proposed approach are discussed.
