  We study the visible photon production from the viscous dissipation of the
dark matter (DM) fluid. The visible photon production depends on the magnitude
of the dark matter viscosity and becomes important at the late times. We argue
that for sufficiently large dark matter viscosity, the number of the resonantly
converted visible photons becomes large which populates the Rayleigh-Jeans (RJ)
tails of the Cosmic Microwave Background (CMB) radiation. Consequently, these
excess visible photons possibly can explain the reported EDGES anomaly in the
21 cm signal. 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.
