In the core of a neutron star we expect to find a dense plasma comprised of electrons, muons, protons and neutrons interacting via electromagnetic and strong forces. The complicated interplay of these interactions has a profound impact on the spectra of photons within the plasma. The in-medium photon in turn is an important ingredient in the calculation of transport coefficients in the outer core of neutron stars which determine the damping of hydrodynamic modes, r-modes and the spin evolution of neutron stars. I will provide a detailed study of the photon spectrum based on the Random Phase Approximation (RPA), placing a particular focus on screening effects and collective modes. Potential applications to the study of dark matter particles (in particular the hypothetical "dark photon") traversing through dense nuclear matter will be discussed.