Equation of State of Dark Matter after Planck Data

In this paper, we loosen the zero equation of state of dark matter to a constant $w_{dm}$. By using the currently available cosmic observations which include the type-Ia supernovae, the baryon acoustic oscillation, the WiggleZ measurements of matter power spectrum and the cosmic microwave background radiation from the firs release of {\it Planck} dat through the Markov chain Monte Carlo method, we found the equation of state of dark matter in $3\sigma$ regions: $w_{dm}=0.000707_{-0.000747-0.00149-0.00196}^{+0.000746+0.00146+0.00193}$. The difference of the minimum ${\chi }^2$ between the $\mathrm{\Lambda }$CDM and $\mathrm{\Lambda }$wDM models is $\mathrm{\Delta }{\chi }_{min}^2=0.446$ for one extra model parameter $w_{dm}$. Although the currently available cosmic observations favor the $\mathrm{\Lambda }$wDM mildly, no significant deviation from the $\mathrm{\Lambda }$CDM model is found in $1\sigma$ regions.