  In this exploratory study, we investigate the bounds on the equation of state
of dark matter. Modeling dark matter as a fluid component, we take into account
both positive and negative fixed equations of state. Using CMB, supernovae Ia
and large scale structure data we find constraints on the equation of state in
a modified LambdaCDM cosmology. We obtain -1.50 x 10^{-6} < w_{dm} < 1.13 x
10^{-6} if the dark matter produces no entropy and -8.78 x 10^{-3}< w_{dm} <
1.86 x 10^{-3} if the adiabatic sound speed vanishes, both at 3 sigma
confidence level.
