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)).
