  Different models of the cosmic substratum which pretend to describe the
present stage of accelerated expansion of the Universe like the $\Lambda$CDM
model or a Chaplygin gas, can be seen as special realizations of a holographic
dark energy cosmology if the option of an interaction between pressurless dark
matter and dark energy is taken seriously. The corresponding interaction
strength parameter plays the role of a cosmological constant. Differences occur
at the perturbative level. In particular, the pressure perturbations are
intrinsically non-adiabatic.
