Astrophysical plasmas can be an abundant source of particles with a small coupling to photons, for instance in dark sectors with a kinetically mixed U(1). In many situations, the decay of plasmons (photons with an in-medium effective mass) is the most efficient process for generating new kinds of particles. In this talk, I will discuss two distinct examples: the production of gravitationally bound particles in the sun and the freeze-in of dark matter in the early Universe. In the former case, I will show how gravitationally bound particles from the sun may be detectable with an experimental setup that coherently deflects the solar MCP wind, generating an oscillating electric field in a shielded detector. In the latter case, I will show that dark matter freeze-in from plasmon decay may be detectable through its cosmological clustering and through a drag effect imparted on the photon-baryon fluid prior to recombination. If there is sufficient time, I will also discuss the detection of axion dark matter decay that is stimulated by the synchrotron radiation coming from the plasma suffusing supernova remnants. In all of these examples, the unique phase space of new particles provides the main observational handle for discovery.