  Motivated by recent claims of a compelling ~3.5 keV emission line from nearby
galaxies and galaxy clusters, we investigate a novel plasma model incorporating
a charge exchange component obtained from theoretical scattering calculations.
Fitting this kind of component with a standard thermal model yields positive
residuals around 3.5 keV, produced mostly by S XVI transitions from principal
quantum numbers n > 8 to the ground. Such high-n states can only be populated
by the charge exchange process. In this scenario, the observed 3.5 keV line
flux in clusters can be naturally explained by an interaction in an effective
volume of ~1 kpc^3 between a ~3 keV temperature plasma and cold dense clouds
moving at a few hundred km/s. The S XVI lines at ~3.5 keV also provide a unique
diagnostic of the charge exchange phenomenon in hot cosmic plasmas.
