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The recently claimed anomaly in the measurement of the 21 cm hydrogenabsorption signal by EDGES at $z\sim 17$, if cosmological, requires theexistence of new physics. The possible attempts to resolve the anomaly rely oneither (i) cooling the hydrogen gas via new dark matter-hydrogen interactionsor (ii) modifying the soft photon background beyond the standard CMB one, aspossibly suggested also by the ARCADE~2 excess. We argue that solutionsbelonging to the first class are generally in tension with cosmological darkmatter probes once simple dark sector models are considered. Therefore, wepropose soft photon emission by light dark matter as a natural solution to the21 cm anomaly, studying a few realizations of this scenario. We find that thesignal singles out a photophilic dark matter candidate characterised by anenhanced collective decay mechanism, such as axion mini-clusters.

The EDGES 21 cm Anomaly and Properties of Dark Matter