  We study the interaction between dark sectors by considering the momentum
transfer caused by the dark matter scattering elastically within the dark
energy fluid. Describing the dark scattering analogy to the Thomson scattering
which couples baryons and photons, we examine the impact of the dark scattering
in CMB observations. Performing global fitting with the latest observational
data, we find that for a dark energy equation of state $w<-1$, the CMB gives
tight constraints on dark matter-dark energy elastic scattering. Assuming a
dark matter particle of proton mass, we derive an elastic scattering cross
section of $\sigma_D < 3.295 \times 10^{-10} \sigma_T$ where $\sigma_T$ is the
cross section of Thomson scattering. For $w>-1$, however, the constraints are
poor. For $w=-1$, $\sigma_D$ can formally take any value.
