Constraining dark matter properties with Cosmic Microwave Background
  observations
  We examine how the properties of dark matter, parameterised by an equation of
state parameter $w$ and two perturbative Generalised Dark Matter (GDM)
parameters $c^2_s$ (the sound speed) and $c^2_\text{vis}$ (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 dark matter model
beyond the Cold Dark Matter (CDM) paradigm. The dark matter equation of state
is constrained to be within $-0.000896<w<0.00238$ at the $99.7\%$ confidence
level, which is several times stronger than constraints found previously using
WMAP data. The parameters $c^2_s$ and $c^2_\text{vis}$ are constrained to be
less than $3.21\times10^{-6}$ and $6.06\times10^{-6}$ respectively at the
$99.7\%$ confidence level. The inclusion of the GDM parameters does
significantly affect the error bars on several $\Lambda$CDM parameters, notably
the dimensionless dark matter density $\omega_g$ and the derived parameters
$\sigma_8$ and $H_0$. This can be partially alleviated with the inclusion of
data constraining the expansion history of the universe.
