  We consider the generalized Chaplygin gas (GCG) proposal for unification of
dark energy and dark matter and show that it admits an unique decomposition
into dark energy and dark matter components once phantom-like dark energy is
excluded. Within this framework, we study structure formation and show that
difficulties associated to unphysical oscillations or blow-up in the matter
power spectrum can be circumvented. Furthermore, we show that the dominance of
dark energy is related to the time when energy density fluctuations start
deviating from the linear $\delta \sim a$ behaviour.
