  Specific internal self-interactions in gaseous cosmic fluids are shown to
give rise to effective negative pressures which may violate the strong energy
condition. On this basis we discuss the transition from an initial de Sitter
phase to a subsequent Friedmann-Lema\^{\i}tre-Robertson-Walker period as a
non-equilibrium configuration of an ultrarelativistic gas. An accelerated
expansion of the present universe is obtained as the consequence of a negative
internal friction force which is self-consistently exerted on the
non-relativistic cold dark matter (CDM) type microscopic constituents of the
cosmic gas.
