  We present a Vector Dark Matter (VDM) model that explains the 3.5 keV line
recently observed in the XMM-Newton observatory data from galaxy clusters. In
this model, dark matter is composed of two vector bosons, $V$ and $V^\prime$,
which couple to the photon through an effective generalized Chern-Simons
coupling, $g_V$. $V^\prime$ is slightly heavier than $V$ with a mass splitting
$m_{V^\prime}-m_V\simeq 3.5$~keV. The decay of $V^\prime$ to $V$ and a photon
gives rise to the 3.5~keV line. The production of $V$ and $V^\prime$ takes
place in the early universe within the freeze-in framework through the
effective $g_V$ coupling when $m_{V^\prime}<T<\Lambda $, $\Lambda$ being the
cut-off above which the effective $g_V$ coupling is not valid. We introduce a
high energy model that gives rise to the $g_V$ coupling at low energies. To do
this, $V$ and $V^\prime$ are promoted to gauge bosons of spontaneously broken
new $U(1)_V$ and $U(1)_{V^\prime}$ gauge symmetries, respectively. The high
energy sector includes milli-charged chiral fermions that lead to the $g_V$
coupling at low energy via triangle diagrams.
