  We investigate the possibility of using cosmological observations to probe
and constrain an imperfect dark energy fluid. We consider a general
parameterization of the dark energy component accounting for an equation of
state, speed of sound and viscosity. We use present and future data from the
cosmic microwave background radiation (CMB), large scale structures and
supernovae type Ia. We find that both the speed of sound and viscosity
parameters are difficult to nail down with the present cosmological data. Also,
we argue that it will be hard to improve the constraints significantly with
future CMB data sets. The implication is that a perfect fluid description might
ultimately turn out to be a phenomenologically sufficient description of all
the observational consequences of dark energy. The fundamental lesson is
however that even then one cannot exclude, by appealing to observational
evidence alone, the possibility of imperfectness in dark energy.
