Generating X-ray lines from annihilating dark matter
  We propose different scenarios where a keV dark matter annihilates to produce a monochromatic signal. The process is generated through the exchange of a light scalar of mass of order 300 keV - 50 MeV coupling to photon through loops or higher dimensional operators. For natural values of the couplings and scales, the model can generate a gamma-ray line which can fit with the recently identified 3.5 keV X-ray line. 
Emilian Dudas, Lucien Heurtier and Yann Mambrini
hep-ph astro-ph.HE hep-th