  We show that the dynamical realization of the acceleration-enlarged Galilean
symmetry leads to nonrelativistic massless particles whose energy may be
negative. We present a fluid mechanical generalisation of this observation and
use it to contruct a nonrelativistic two-dimensional fluid model which
possesses solutions with a negative energy density. Considering this model as
describing dark energy in a baby universe (two space dimensions) we show that
its negative energy density leads to a repulsive gravitational interaction of
the fluid with any test body.
