  The EDGES Collaboration has recently announced the detection of the 21-cm
spectrum with an absorption profile centred at $78$ megahertz, of which the
depth is deeper than that expected by the standard cosmological paradigm. To
enrich the heating process of baryons due to scattering with dark matter during
dark ages, we in this Letter explore the possibility of extra heat transfer
between dark sector compositions and their observational signatures on the
21-cm cosmological spectrum. By parameterizing interaction models of the dark
Universe, we find that the observational constraint on the parameter space of
dark matter can be slightly relaxed but the discrepancy with the commonly
predicted parameter space of weakly interacting massive particles remains. Our
analyses also reveal that the interaction between dark compositions may leave
observational signatures on the 21-cm spectrum during dark ages and thus would
become detectable in the forthcoming 21-cm cosmology.
