The nature of dark matter is one of the most thrilling riddles for both cosmology and particle physics nowadays .
While in the typical models the dark sector is composed only by weakly interacting massive particles , an arguably more natural scenario would include a whole set of gauge interactions which are invisible for the standard model but that are in contact with the dark matter .
We present a method to constrain the number of massless gauge bosons and other relativistic particles that might be present in the dark sector using current and future cosmic microwave background data , and provide upper bounds on the size of the dark sector .
We use the fact that the dark matter abundance depends on the strength of the interactions with both sectors , which allows one to relate the freeze-out temperature of the dark matter with the temperature of this cosmic background of dark gauge bosons .
This relation can then be used to calculate how sizable is the impact of the relativistic dark sector in the number of degrees of freedom of the early Universe , providing an interesting and testable connection between cosmological data and direct/indirect detection experiments .
The recent Planck data , in combination with other cosmic microwave background experiments and baryonic acoustic oscillations data , constrains the number of relativistic dark gauge bosons , when the freeze-out temperature of the dark matter is larger than the top mass , to be N \lesssim 14 for the simplest scenarios , while those limits are slightly relaxed for the combination with the Hubble constant measurements to N \lesssim 20 .
Future releases of Planck data are expected to reduce the uncertainty by approximately a factor 3 , what will reduce significantly the parameter space of allowed models .