  Dark photons (DPs) produced in the early Universe are well-motivated dark
matter (DM) candidates. We show that the recently proposed tunable plasma
haloscopes are particularly advantageous for DP searches. While in-medium
effects suppress the DP signal in conventional searches, plasma haloscopes make
use of metamaterials that enable resonant absorption of the DP by matching its
mass to a tunable plasma frequency and thus enable efficient plasmon
production. Using thermal field theory, we confirm the in-medium DP absorption
rate within the detector. This scheme allows to competitively explore a
significant part of the DP DM parameter space in the DP mass range of $6-400$
$\mu$eV. If a signal is observed, the observation of a daily or annual
modulation of the signal would be crucial to clearly identify the signal as due
to DP DM and could shed light on the production mechanism.
