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 $\Lambda$CDM and $\Lambda$wDM
models is $\Delta\chi^2_{min}=0.446$ for one extra model parameter $w_{dm}$.
Although the currently available cosmic observations favor the $\Lambda$wDM
mildly, no significant deviation from the $\Lambda$CDM model is found in
$1\sigma$ regions.
