  In a dense cloud of massive fermions interacting by exchange of a light
scalar field, the effective mass of the fermion can become negligibly small. As
the cloud expands, the effective mass and the total energy density eventually
increase with decreasing density. In this regime, the pressure-density relation
can approximate that required for dark energy. We apply this phenomenon to the
expansion of the Universe with a very light scalar field and infer relations
between the parameters available and cosmological observations. Majorana
neutrinos at a mass that may have been recently determined, and fermions such
as the Lightest Supersymmetric Particle (LSP) may both be consistent with
current observations of dark energy.
