Constraining the properties of dark matter with observations of the cosmic microwave background

We examine how the properties of dark matter, parameterized by an equation-of-state parameter w and two perturbative generalized dark matter (GDM) parameters, c ² _s (the sound speed) and $c_{vis}^2$ (the viscosity), are constrained by existing cosmological data, particularly the Planck 2015 data release. We find that the GDM parameters are consistent with zero, and are strongly constrained, showing no evidence for extending the model of dark matter beyond the cold dark matter (CDM) paradigm. The equation of state of dark matter is constrained to be within $-0.000896<w<0.00238$ at the 99.7% confidence level (CL), which is several times stronger than constraints found previously using WMAP data. The parameters c ² _s and $c_{vis}^2$ are constrained to be less than $3.21\times {10}^{-6}$ and 6.06 × 10⁻⁶ respectively at the 99.7% CL. The inclusion of the GDM parameters does significantly affect the error bars on several ΛCDM parameters, notably the dimensionless dark matter density ω _g and the derived parameters σ ₈ and H ₀. This can be partially alleviated with the inclusion of data constraining the expansion history of the universe.