  We discuss the 3.55 keV X-ray line anomaly reported by XMN-Newton X-ray
observatory using data of various galaxy clusters and Andromeda galaxy in a
radiative neutrino model, in which the mixing between the active neutrino and
the dark matter is generated at two-loop level after the spontaneous breaking
of $Z_2$ symmetry. It might provide us a natural explanation of its tiny mixing
${\cal O}(10^{-10})$, which is observed by their experiments. Such an Abelian
discrete symmetry plays a crucial role in differentiating the TeV scale
Majorana field from our dark matter, whose mass is expect to be around 7.1 keV.
