  We consider a simple class of models where dark radiation has
self-interactions and therefore does not free stream. Such dark radiation has
no anisotropic stress (or viscosity), leaving a distinct signature on the CMB
angular power spectrum. Specifically we study a possibility that hidden gauge
bosons and/or chiral fermions account for the excess of the effective number of
neutrino species. They have gauge interactions and remain light due to the
unbroken hidden gauge symmetry, leading to \Delta N_{\rm eff} \simeq 0.29 in
some case.
