  Assuming that the Universe is filled by one single fluid, we present in the
context of General Relativity a possible explanation for the acceleration of
the Universe. We use ordinary thermodynamics and the fact that small matter
perturbations barely propagate in our Universe, to derive a general solution
for a single fluid in which the speed of sound vanishes. We find a model that
contains $\Lambda$CDM as a special case, and is compatible with current
observational data.
