  The Cold Dark Matter (CDM) model, wherein the dark matter is treated as a
pressureless perfect fluid, provides a good fit to galactic and cosmological
data. With the advent of precision cosmology, it should be asked whether this
simplest model needs to be extended, and whether doing so could improve our
understanding of the properties of dark matter. One established
parameterisation for generalising the CDM fluid is the Generalised Dark Matter
(GDM) model, in which dark matter is an imperfect fluid with pressure and shear
viscosity that fulfill certain closure equations. We investigate these closure
equations and the three new parametric functions they contain: the background
equation of state w, the speed of sound c_s^2 and the viscosity c_{vis}^2.
Taking these functions to be constant parameters, we analyse an exact solution
of the perturbed Einstein equations in a GDM-dominated universe and discuss the
main effects of the three parameters on the Cosmic Microwave Background (CMB).
Our analysis suggests that the CMB alone is not able to distinguish between the
GDM sound speed and viscosity parameters, but that other observables, such as
the matter power spectrum, are required to break this degeneracy. We also
consider other descriptions of imperfect fluids that have a non-perturbative
definition and relate these to the GDM model. In particular, we consider scalar
fields, an effective field theory (EFT) of fluids, an EFT of Large Scale
Structure, non-equilibrium thermodynamics and tightly-coupled fluids. These
descriptions could be used to extend the GDM model into the nonlinear regime of
structure formation, which is necessary if the wealth of data available on
those scales is to be employed in constraining the model. We also derive the
initial conditions for adiabatic and isocurvature perturbations and provide the
result in a form ready for implementation in Einstein-Boltzmann solvers.
