Could the 21-cm absorption be explained by the dark matter suggested by
  $^8$Be transitions?
  The stronger than expected 21-cm absorption was observed by EDGES recently,
and another anomaly of $^8$Be transitions would be signatures of new
interactions. These two issues may be related to each other, e.g., pseudoscalar
$A$ mediated fermionic millicharged dark matter (DM), and the 21-cm absorption
could be induced by photon mediated scattering between MeV millicharged DM and
hydrogen. This will be explored in this paper. For fermionic millicharged DM
$\bar{\chi} \chi$ with masses in a range of $2 m_A < 2 m_{\chi} < 3 m_A$, the
p-wave annihilation $\bar{\chi} \chi \to A A$ would be dominant during DM
freeze-out. The s-wave annihilation $\bar{\chi} \chi$ $\to A, \gamma $ $\to e^+
e^-$ is tolerant by constraints from CMB and the 21-cm absorption. The
millicharged DM can evade constraints from direct detection experiments. The
process of $K^+ \to \pi^+ \pi^0$ with the invisible decay $\pi^0 \to \bar{\chi}
\chi$ could be employed to search for the millicharged DM, and future high
intensity $K^+$ sources, such as NA62, will do the job.
