  The recently claimed anomaly in the measurement of the 21 cm hydrogen
absorption signal by EDGES at $z\sim 17$, if cosmological, requires the
existence of new physics. The possible attempts to resolve the anomaly rely on
either (i) cooling the hydrogen gas via new dark matter-hydrogen interactions
or (ii) modifying the soft photon background beyond the standard CMB one, as
possibly suggested also by the ARCADE~2 excess. We argue that solutions
belonging to the first class are generally in tension with cosmological dark
matter probes once simple dark sector models are considered. Therefore, we
propose soft photon emission by light dark matter as a natural solution to the
21 cm anomaly, studying a few realizations of this scenario. We find that the
signal singles out a photophilic dark matter candidate characterised by an
enhanced collective decay mechanism, such as axion mini-clusters.
