Viscous dark matter and its implication for 21 cm signal

We explore the effect of the dark matter (DM) viscosity on the cosmic evolution. The DM viscosity dissipates energy and increases the DM temperature. We propose that the energy dissipation from the Viscous dark matter, can generate photons that populate the low energy tail of the Cosmic Microwave Background (CMB) radiation. We argue that the produced photons are large enough to explain the anomaly in the 21 cm global signal observed by the EDGES collaboration.