Abstract
In this work, we study the visible photon production from the viscous dissipation of the dark matter (DM) fluid. The generation of visible photons depends on the magnitude of the dark matter viscosity and becomes important at the late time. We argue that for sufficiently large dark matter viscosity, the number of resonantly converted visible photons becomes large which populates the Rayleigh-Jeans (RJ) tail of the Cosmic Microwave Background (CMB) radiation. Consequently, these excess visible photons can explain the anomaly in the 21 cm signal observed by the EDGES collaboration. Further, we explore the parameter space for which the 21 cm signal can provide the region to probe the dark radiation and the DM viscosity.