  The unidentified emission line at the energy of $\sim$3.5~keV observed in
X-rays from galaxy clusters may originate from a process involving a dark
matter particle. On the other hand, a weakly interacting massive particle
(WIMP) has been an attractive dark matter candidate, due to its well-understood
thermal production mechanism and its connection to physics at the TeV scale. In
this paper, we pursue the possibility that the 3.5-keV X-ray arises from a late
time decay of a WIMP dark matter into another WIMP dark matter, both of which
have the mass of $O(100)$~GeV and whose mass splitting is about 3.5~keV. We
focus on the simplest case where there are two Majorana dark matter particles
and two charged scalars that couple with a standard model matter particle. By
assuming a hierarchical structure in the couplings of the two dark matter
particles and two charged scalars, it is possible to explain the 3.5-keV line
and realize the WIMP dark matter scenario at the same time. Since the effective
coupling of the two different Majorana dark matter particles and one photon
violates CP symmetry, the model always contains a new source of CP violation,
so the model's connection to the physics of electric dipole moments is
discussed. The model's peculiar signatures at the LHC are also studied. We show
the prospect of detecting the charged scalars through a detailed collider
simulation.
