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 \times 10 ^ { -6 } and 6.06 \times 10 ^ { -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 .