  We present a new mechanism for producing the correct relic abundance of dark
photon dark matter over a wide range of its mass, extending down to
$10^{-20}\,\mathrm{eV}$. The dark matter abundance is initially stored in an
axion which is misaligned from its minimum. When the axion starts oscillating,
it efficiently transfers its energy into dark photons via a tachyonic
instability. If the dark photon mass is within a few orders of magnitude of the
axion mass, $m_{\gamma'}/m_a = {\cal O}(10^{-3} - 1)$, then dark photons make
up the dominant form of dark matter today. We present a numerical lattice
simulation for a benchmark model that explicitly realizes our mechanism. This
mechanism firms up the motivation for a number of experiments searching for
dark photon dark matter.
