In this talk, I investigate the new mathematical approximation for the equation of state of unified dark matter and dark energy. This approximation provides well-behaving evolution of state for both small and high redshifts. In particular, the dark fluid composed by dark matter and dark energy described by our approximation behaves like dark matter for big redshifts and like dark energy at low redshifts. Hence in our approximation, one can parameterize dark energy and dark matter using a single dark fluid, like in the case of the Chaplygin gas. Within this approximation, we consider two different cases: one with dark energy equation of state parameter fixed to −1 and the second one, where w≠−1 is chosen to match the best fit to the data. We study main cosmological properties of these cases at the expansion and perturbation levels. Based on Markov chain Monte Carlo method with currently available cosmic observational data sets, we constrain these models to determine the cosmological parameters at the level of background and clustering of matter. We consider the interaction between dark matter and dark energy which directly affects the evolution of matter and its clustering. Our model appears to be perfectly consistent with the concordance ΛCDM universe, while providing unification of dark energy and dark matter.