  A fluid interpretation of Cardassian expansion is developed. Here, the
Friedmann equation takes the form $H^2 = g(\rho_M)$ where $\rho_M$ contains
only matter and radiation (no vacuum). The function $g(\rhom)$ returns to the
usual $8\pi\rhom/(3 m_{pl}^2)$ during the early history of the universe, but
takes a different form that drives an accelerated expansion after a redshift $z
\sim 1$. One possible interpretation of this function (and of the right hand
side of Einstein's equations) is that it describes a fluid with total energy
density $\rho_{tot} = {3 m_{pl}^2 \over 8 \pi} g(\rhom) = \rhom + \rho_K$
containing not only matter density (mass times number density) but also
interaction terms $\rho_K$. These interaction terms give rise to an effective
negative pressure which drives cosmological acceleration. These interactions
may be due to interacting dark matter, e.g. with a fifth force between
particles $F \sim r^{\alpha -1}$. Such interactions may be intrinsically four
dimensional or may result from higher dimensional physics. A fully relativistic
fluid model is developed here, with conservation of energy, momentum, and
particle number. A modified Poisson's equation is derived. A study of
fluctuations in the early universe is presented, although a fully relativistic
treatment of the perturbations including gauge choice is as yet incomplete.
