  We review the implications of having a non-trivial matter component in the
universe and the potential for detecting such a component through the matter
power spectrum and ISW effect. We adopt a phenomenological approach and
consider the mysterious dark energy to be a cosmic fluid. It is thus fully
characterized, up to linear order, by its equation of state and its speed of
sound. Whereas the equation of state has been widely studied in the literature,
less interest has been devoted to the speed of sound. Its observational
consequences come predominantly from very large scale modes of dark matter
perturbations (k < 0.01 h.Mpc^{-1}). Since these modes have hardly been probed
so far by large scale galaxy surveys, we investigate whether joint constraints
that can be placed on those two quantities using the recent CMB fluctuations
measurements by WMAP as well as the recently measured CMB large scale structure
cross-correlation. We find only a tentative 1$sigma$ detection of the speed of
sound, from CMB alone, c_{s}^{2}<0.04 at this low significance level.
Furthermore, the current uncertainties in bias in the matter power spectrum
preclude any constraints being placed using the cross correlation of CMB with
the NVSS radio survey. We believe however that improvements in bias through
improved survey scales and depths in the near future will improve hopes of
detecting the speed of sound.
