  Many extensions of Standard Model (SM) include a dark sector which can
interact with the SM sector via a light mediator. We explore the possibilities
to probe such a dark sector by studying the distortion of the CMB spectrum from
the blackbody shape due to the elastic scatterings between the dark matter and
baryons through a hidden light mediator. We in particular focus on the model
where the dark sector gauge boson kinetically mixes with the SM and present the
future experimental prospect for a PIXIE-like experiment along with its
comparison to the existing bounds from complementary terrestrial experiments.
