  In this work we model galactic halos describing the dark matter as a non zero
pressure fluid and derive, not impose, a dark matter equation of state by using
observational data of the rotation curves of galaxies. In order to reach
hydrostatic equilibrium, as expected for the halo, it is mandatory that dark
fluid's pressure should not be zero. The equation of state is obtained by
solving the matter-geometry system of equations assuming different dark matter
density or rotational velocity profiles. The resulting equations of state are,
in general, different to a barotropic equation of state. The free parameters of
the equation of state are fixed by fitting the observed rotational velocities
of a set of galaxies.
