  We show that a de Sitter spacetime is a solution of Einstein's field
equations with the energy momentum tensor of a self-interacting, classical
Maxwell-Boltzmann gas in collisional equilibrium. The self-interaction is
described by a four-force which is quadratic in the (spatially projected)
particle four-momenta. This force does not preserve the particle number and
gives rise to an exponential increase in the comoving entropy of the universe
while the temperature of the latter remains constant. These properties of a gas
universe are related to the existence of a ``projector-conformal'' timelike
Killing vector representing a symmetry which is ``in between'' the symmetries
characterized by a Killing vector and those characterized by a conformal
Killing vector.
