  We propose a novel mechanism enabling us to have a continuum bump as a
signature of gamma-ray excess in indirect detection experiments of dark matter
(DM), postulating a generic dark sector having (at least) two DM candidates.
With the assumption of non-zero mass gap between the two DM candidates, the
heavier one directly communicates to the partner of the lighter one. Such a
partner then decays into a lighter DM particle along with an "axion-like"
particle (ALP) or dark "pion", which subsequently decays into a pair of
photons, via a more-than-one step cascade decay process. Since the cascade is
initiated by the dark partner obtaining a non-trivial fixed boost factor, a
continuum gamma-ray energy spectrum naturally arises even with a particle
directly decaying into two photons. We apply the main idea to the energy
spectrum of the GeV gamma-rays from around the Galactic Center (GC), and find
that the relevant observational data is well-reproduced by the theory
expectation predicted by the proposed mechanism. Remarkably, the relevant
energy spectrum has a robust peak at half the mass of the ALP or dark pion, as
opposed to popular DM models directly annihilating to Standard Model particles
where physical interpretations of the energy peak are not manifest. Our data
analysis reports substantially improved fits, compared to those annihilating DM
models, and ~900 MeV mass of the ALP or dark pion.
