We know that an electron and a positron can apparently annihilate each other in space, and that two gamma ray photons emerge from the site. Most physicists are happy to settle for this narrative without enquiring too deeply into the close-up details of the metamorphosis process. These however are the very details which are of interest as regards identifying the nature of the energy that is stored in space. The relationship, h = cke, where h is Planck's constant, c is the speed of light, k is the Boltzmann constant, and e is the charge of an electron, was brought to my attention seven years ago by Antonio Saraiva from Portugal. Unless this numerical product is a major coincidence without meaning, then it is telling us something about the physical nature of the medium for the propagation of light.
These two linked articles below provide a solution to the Planck-Einstein relation, E = hf, by explaining photon energy as lying in the vestibule of the atom, whereas Planck's constant itself is a characteristic of the elasticity of the luminiferous medium. The photon theory of light is therefore just a packaging theory akin to shipping container theory. It bears no relationship to the deeper underlying contents. It bears no relationship to the deeper and more fundamental wave theory of light.