  Universe evolution, as described by Friedmann's equations, is determined by
source terms fixed by the choice of pressure $\times$ energy-density equations
of state $p(\rho)$. The usual approach in Cosmology considers equations of
state accounting only for kinematic terms, ignoring the contribution from the
interactions between the particles constituting the source fluid. In this work
the importance of these neglected terms is emphasized. A systematic method,
based on the Statistical Mechanics of real fluids, is proposed to include them.
A toy-model is presented which shows how such interaction terms can engender
significant cosmological effects.
