  We propose a scenario where the saxion dominates the energy density of the
Universe and reheats the standard model sector via the dilatonic coupling,
while its axionic partner contributes to dark matter decaying into photons via
the same operator in supersymmetry. Interestingly, for the axion mass $m_a
\simeq 7$keV and the decay constant $f_a \simeq 10^{14-15}$GeV, the recently
discovered X-ray line at $3.5$keV in the XMM Newton X-ray observatory data can
be explained. We discuss various cosmological aspects of the $7$keV axion dark
matter such as the production of axion dark matter, the saxion decay process,
hot dark matter and isocurvature constraints on the axion dark matter, and the
possible baryogenesis scenarios.
