  Given that the dark matter and dark energy in the universe affect
cosmological observables only gravitationally, their phenomenology may be
described by a single stress energy tensor. True unification however requires a
theory that reproduces the successful phenomenology of LCDM and that
requirement places specific constraints on the stress structure of the matter.
We show that a recently proposed unification through an offset quadratic
kinetic term for a scalar field is exactly equivalent to a fluid with a
closed-form barotropic equation of state plus cosmological constant. The finite
pressure at high densities introduces a cutoff in the linear power spectrum,
which may alleviate the dark matter substructure problem; we provide a
convenient fitting function for such studies. Given that sufficient power must
remain to reionize the universe, the equation of state today is nonrelativistic
with p proportional to rho^2 and a Jeans scale in the parsec regime for all
relevant densities. Structure may then be evolved into the nonlinear regime
with standard hydrodynamic techniques. In fact, the model is equivalent to the
well-studied collisional dark matter with negligible mean free path. If recent
observations of the triaxiality of dark matter halos and ram pressure stripping
in galaxy clusters are confirmed, this model will be ruled out.
