  Previous quantum field estimations of the QCD vacuum in the expanding
space-time lead to a dark energy component scaling linearly with the Hubble
parameter, which gives the correct figure for the observed cosmological term.
Here we show that this behaviour also appears at the classical level, as a
result of the chiral symmetry breaking in a low energy, effective
$\sigma$-model. The dark sector is described in a unified way by the $\sigma$
condensate and its fluctuations, giving rise to a decaying dark energy and a
homogeneous creation of non-relativistic dark particles. The creation rate and
the future asymptotic de Sitter horizon are both determined by the $\sigma$
mass scale.
