  Recently the EDGES experiment reported an enhanced 21cm absorption signal in
the radio wave observation, which may be interpreted as either anomalous
cooling of baryons or heating of cosmic microwave background photons. In this
paper, we pursue the latter possibility. We point out that dark radiation
consisting of axion-like particles can resonantly convert into photons under
the intergalactic magnetic field, which can effectively heat up the radiation
in the frequency range relevant for the EDGES experiment. This may explain the
EDGES anomaly.
