  In the past decade, some telescopes (e.g. Fermi-LAT, AMS and DAMPE) were
launched to detect the signals of annihilating dark matter in our Galaxy.
Although some excess of gamma rays, anti-protons and electrons/positrons have
been reported and claimed as dark matter signals, the uncertainties of Galactic
pulsars' contributions are still too large to confirm the claims. In this
letter, we report a possible radio signal of annihilating dark matter
manifested in the archival radio continuum spectral data of the Abell 4038
cluster. By assuming the thermal annihilation cross section and comparing the
dark matter annihilation model with the null hypothesis (cosmic ray emission
without dark matter annihilation), we get very large Test Statistic values TS
$>45$ for four popular annihilation channels, which correspond to more than
$6.5 \sigma$ statistical preference. This provides a very strong evidence for
the existence of annihilating dark matter. In particular, our results also
support the recent claims of dark matter mass $m \approx 30-50$ GeV
annihilating via the $b\bar{b}$ quark channel with the thermal annihilation
cross section.
