Laboratory measurements compellingly supports a charge-exchange mechanism for the “Dark matter” ∼ 3.5 keV X-ray line

Synopsis The reported observations of an unidentified X-ray line feature at ∼ 3.5 keV from galaxy clusters have driven a lively discussion about its possible dark matter origin. Motivated by this, we have investigated the X-ray spectra of highly ionized bare sulfur ions following charge exchange with residual gas in the electron beam ion trap, as a source or a contributor to this X-ray line. The X-ray feature at about 3.5 keV shows up in the experiment, which could explain the the astrophysical observations and confirm the predictions of Gu et al.