  To describe dark energy we introduce a fluid model with no free parameter on
the microscopic level. The constituents of this fluid are massless particles
which are a dynamical realisation of the unextended $D=(3+1)$ Galilei algebra.
These particles are exotic as they live in an enlarged phase space. Their only
interaction is with gravity. A minimal coupling to the gravitational field,
satisfying Einstein's equivalence principle, leads to a dynamically active
gravitational mass density of either sign. A two-component model containing
matter (baryonic and dark) and dark energy leads, through the cosmological
principle, to Friedmann-like equations. Their solutions show a deceleration
phase for the early universe and an acceleration phase for the late universe.
We predict the Hubble parameter $H(z)/H_0$ and the deceleration parameter
$q(z)$ and compare them with available experimental data. We also discuss a
reduced model (one component dark sector) and the inclusion of radiation. Our
model shows no stationary modification of Newton's gravitational potential.
