Axion

Warm dark matter

Physics

Dark matter

Cosmic microwave background

Dark fluid

Hot dark matter

Scalar field dark matter

Particle physics

Anomaly (physics)

Photon

Dark energy

Cosmology

Light dark matter

Astrophysics

Quantum mechanics

Anisotropy

Physics Letters B

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.

The EDGES 21 cm anomaly and properties of dark matter