  We update a previous investigation of cosmological effects of a non-standard
interaction between neutrinos and dark matter. Parameterizing the
elastic-scattering cross section between the two species as a function of the
temperature of the universe, the resulting neutrino-dark matter fluid has a
non-zero pressure, which determines diffusion-damped oscillations in the matter
power spectrum similar to the acoustic oscillations generated by the
photon-baryon fluid. Using cosmic microwave background data in combination with
large scale structure experiment results, we then put constraints on the
fraction of the interacting dark matter component as well as on the
corresponding opacity.
