  We propose a scenario in which extra light fermions in an $E_6$-inspired U(1)
extension of the standard model constitute the dark matter, as a simple
variation of our model for dark radiation presented in 2010. Interestingly, for
the light fermions of mass about 7 keV, they radiatively decay into active
neutrinos and photons with a lifetime in the range of $10^{27}-10^{28}$
seconds, which naturally explains the recently discovered 3.5 keV X-ray line
signal.
