New emission line at ~3.5 keV - observational status, connection with
  radiatively decaying dark matter and directions for future studies
  Recent works of [1402.2301,1402.4119], claiming the detection of extra
emission line with energy ~3.5 keV in X-ray spectra of certain clusters of
galaxies and nearby Andromeda galaxy, have raised considerable interest in
astrophysics and particle physics communities. A number of new observational
studies claim detection or non-detection of the extra line in X-ray spectra of
various cosmic objects. In this review I summarize existing results of these
studies, overview possible interpretations of the extra line, including
intriguing connection with radiatively decaying dark matter, and show future
directions achievable with existing and planned X-ray cosmic missions.
