  A phenomenological approach is proposed to the problem of universe
accelerated expansion and of the dark energy nature. A general class of models
is introduced whose energy density depends on the redshift $z$ in such a way
that a smooth transition among the three main phases of the universe evolution
(radiation era, matter domination, asymptotical de Sitter state) is naturally
achieved. We use the estimated age of the universe, the Hubble diagram of Type
Ia Supernovae and the angular size - redshift relation for compact and
ultracompact radio structures to test whether the model is in agreement with
astrophysical observation and to constrain its main parameters. Although
phenomenologically motivated, the model may be straightforwardly interpreted as
a two fluids scenario in which the quintessence is generated by a suitably
chosen scalar field potential. On the other hand, the same model may also be
read in the context of unified dark energy models or in the framework of
modified Friedmann equation theories.
