  The detection of an unidentified emission line in X-ray spectra of cosmic
objects would be a 'smoking gun' signature for particle physics beyond the
Standard Model. More than a decade of its extensive searches results in several
narrow faint emission lines reported at 3.5, 8.7, 9.4 and 10.1 keV. The most
promising of them is the emission line at ~3.5 keV reported in spectra of
several nearby galaxies and galaxy clusters. Here I summarize its up-to-date
status, overview its possible interpretations, including an intriguing
connection with radiatively decaying dark matter, and outline future directions
for its studies.
