Laboratory measurements compellingly support charge-exchange mechanism   for the 'dark matter' $\sim$3.5 keV X-ray line
  The reported observations of an unidentified X-ray line feature at $\sim$3.5 keV have driven a lively discussion about its possible dark matter origin. Motivated by this, we have measured the \emph{K}-shell X-ray spectra of highly ionized bare sulfur ions following charge exchange with gaseous molecules in an electron beam ion trap, as a source of or a contributor to this X-ray line. We produce $\mathrm{S}^{16+}$ and $\mathrm{S}^{15+}$ ions and let them capture electrons in collision with those molecules with the electron beam turned off while recording X-ray spectra. We observed a charge-exchanged-induced X-ray feature at the Lyman series limit (3.47 $\pm$ 0.06 keV). The inferred X-ray energy is in full agreement with the reported astrophysical observations and supports the novel scenario proposed by Gu and Kaastra (A \& A \textbf{584}, {L11} (2015)). 
Chintan Shah (1), Stepan Dobrodey (1), Sven Bernitt (1 and 2), Ren\'e   Steinbr\"ugge (1), Jos\'e R. Crespo L\'opez-Urrutia ...
astro-ph.HE astro-ph.CO astro-ph.GA physics.atom-ph physics.plasm-ph