  As is known above 90% of the energy content in Universe is made of unknown
dark component. Usually this dark fluid is separated into two parts: dark
matter and dark energy. However, it may be a mixture of these two energy
components, or just one exotic unknown fluid. This property is dubbed as dark
degeneracy. With this motivation, in this paper, a unified dark fluid having
constant adiabatic sound speed $c_s^2=\alpha$, which is in the range $[0,1]$,
is studied. At first, via the energy conservation equation, its energy density,
$\rho_d/\rho_{d0}=(1-B_s)+B_s a^{-3(1+\alpha)}$ where $B_s$ is related to
integration constant from energy conservation equation as another model
parameter, is presented. Then by using Markov Chain Monte Carlo method with
currently available cosmic observational data sets which include type Ia
supernova Union 2, baryon acoustic oscillation and WMAP 7-year data of cosmic
background radiation, we show that small values of $\alpha$ are favored in this
unified dark fluid model. Furthermore, we show that smaller values of
$\alpha<10^{-5}$ are required to match matter (baryon) power spectrum from SDSS
DR7.
