  We propose an alternative mechanism based upon dark matter (DM)
interpretation for anomalous peak signatures in cosmic ray measurements,
assuming an extended dark sector with two DM species. This is contrasted with
previous effort to explain various line-like cosmic-ray excesses in the context
of DM models where the relevant DM candidate directly annihilates into Standard
Model (SM) particles. The heavier DM is assumed to annihilate to an on-shell
intermediate state. As the simplest choice, it decays directly into the lighter
DM along with an unstable particle which in turn decays to a pair of SM states
corresponding to the interesting cosmic anomaly. We show that a sharp continuum
energy peak can be readily generated under the proposed DM scenario, depending
on dark sector particle mass spectra. Remarkably, such a peak is robustly
identified as half the mass of the unstable particle. Furthermore, other
underlying mass parameters are analytically related to the shape of energy
spectrum. We apply this idea to the two well-known line excesses in the cosmic
photon spectrum: 130 GeV gamma-ray line and 3.5 keV X-ray line. Each observed
peak spectrum is well-reproduced by theoretical expectation predicated upon our
suggested mechanism, and moreover, our resulting best fits provide rather
improved chi-square values.
