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The Cold Dark Matter (CDM) model, wherein the dark matter is treated as apressureless perfect fluid, provides a good fit to galactic and cosmologicaldata. With the advent of precision cosmology, it should be asked whether thissimplest model needs to be extended, and whether doing so could improve ourunderstanding of the properties of dark matter. One establishedparameterisation for generalising the CDM fluid is the Generalised Dark Matter(GDM) model, in which dark matter is an imperfect fluid with pressure and shearviscosity that fulfill certain closure equations. We investigate these closureequations and the three new parametric functions they contain: the backgroundequation 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 solutionof the perturbed Einstein equations in a GDM-dominated universe and discuss themain effects of the three parameters on the Cosmic Microwave Background (CMB).Our analysis suggests that the CMB alone is not able to distinguish between theGDM sound speed and viscosity parameters, but that other observables, such asthe matter power spectrum, are required to break this degeneracy. We alsoconsider other descriptions of imperfect fluids that have a non-perturbativedefinition and relate these to the GDM model. In particular, we consider scalarfields, an effective field theory (EFT) of fluids, an EFT of Large ScaleStructure, non-equilibrium thermodynamics and tightly-coupled fluids. Thesedescriptions could be used to extend the GDM model into the nonlinear regime ofstructure formation, which is necessary if the wealth of data available onthose scales is to be employed in constraining the model. We also derive theinitial conditions for adiabatic and isocurvature perturbations and provide theresult in a form ready for implementation in Einstein-Boltzmann solvers.

An extensive investigation of the Generalised Dark Matter model