  We propose a cosmological dark matter production mechanism in the form of a
longitudinal massive vector boson. We build upon the work of Graham et.al.
including non-minimal couplings of the massive vector with gravity, developing
a well motivated set-up from an effective field theory perspective. We
carefully track the dynamics of vector field in passing from inflation to
radiation dominated universe to show that the late time abundance of
longitudinal modes -- excited initially by the quantum fluctuations during
inflation -- can provide the observed dark matter abundance for sufficiently
weak non-minimal coupling and wide range of vector masses $5 \times 10^{-7}
\lesssim m\, [{\rm eV}] \lesssim 5 \times 10^{3}$. The final abundance of dark
matter depends on two parameter, the vector mass and its non-minimal coupling
with gravity. We discuss experimental venues to probe this framework, including
the production of a stochastic gravitational wave background. The latter is
especially interesting, as the same mechanism that generates dark matter can
potentially lead to the production of gravitational waves in the LISA frequency
band, through the second-order effects of large dark matter iso-curvature
perturbations at small scales. We take a first step in this direction,
identifying the potential information that gravitational wave experiments can
provide on the parameter space of dark matter within this scenario.
