  Gamma-rays induced by annihilation or decay of dark matter can be its smoking
gun signature. In particular, gamma-rays generated by internal bremsstrahlung
of Majorana and real scalar dark matter is promising since it can be a leading
emission of sharp gamma-rays. However in the case of Majorana dark matter, its
cross section for internal bremsstrahlung cannot be large enough to be observed
by future gamma-ray experiments if the observed relic density is assumed to be
thermally produced. In this paper, we introduce some degenerate particles with
Majorana dark matter, and show they lead enhancement of the cross section. As a
result, increase of about one order of magnitude for the cross section is
possible without conflict with the observed relic density, and it would be
tested by the future gamma-ray experiments such as GAMMA-400 and Cherenkov
Telescope Array (CTA). In addition, the constraints of perturbativity, positron
observation by the AMS experiment and direct search for dark matter are
discussed.
