  We consider a Friedmann-Robertson-Walker universe with a fluid source obeying
a non-ideal equation of state with "asymptotic freedom," namely ideal gas
behavior (pressure changes directly proportional to density changes) both at
low and high density regimes, following a fluid dynamical model due to Shan and
Chen. It is shown that, starting from an ordinary energy density component,
such fluids naturally evolve towards a universe with a substantial "dark
energy" component at the present time, with no need of invoking any
cosmological constant. Moreover, we introduce a quantitative indicator of
darkness abundance, which provides a consistent picture of the actual
matter-energy content of the universe.

General Relativity and Quantum Cosmology