  An emission line with energy of $E\sim 3.5$ keV has been observed in galaxy
clusters by two experiments. The emission line is consistent with the decay of
a dark matter particle with a mass of $\sim 7$ keV. In this work we discuss the
possibility that the dark particle responsible for the emission is a real
scalar ($\rho$) which arises naturally in a $U(1)_X$ Stueckelberg of MSSM. In
the MSSM Stueckelberg extension $\rho$ couples only to other scalars carrying a
$U(1)_X$ quantum number. Under the assumption that there exists a vectorlike
leptonic generation carrying both $SU(2)_L\times U(1)_Y$ and $U(1)_X$ quantum
numbers, we compute the decay of the $\rho$ into two photons via a triangle
loop involving scalars. The relic density of the $\rho$ arises via the decay
$H^0\to h^0+ \rho$ at the loop level involving scalars, and via the
annihilation processes of the vectorlike scalars into $\rho + h^0$. It is shown
that the galactic data can be explained within a multicomponent dark matter
model where the 7 keV dark matter is a subdominant component constituting only
$(1-10)$\% of the matter relic density with the rest being supersymmetric dark
matter such as the neutralino. Thus the direct detection experiments remain
viable searches for WIMPs. The fact that the dark scalar $\rho$ with no
interactions with the standard model particles arises from a Stueckelberg
extension of a hidden $U(1)_X$ implies that the 3.5 KeV galactic line emission
is a signal from the hidden sector.
