If photons have substance then we can calculate their associated mass by using Planck’s formula in combination with that of Einstein. For photons in the visible region, a frequency of about 1014 cycles per second, this turns out to be about 10-33 grams. We did not know in 1905 that an electron has a mass of about 10-27 grams. we know now that this would make the associated mass of such a photon about one millionth that of an electron.
Gamma rays, with frequencies over 1020 cycles per second arise from within the nucleus and represent a small fraction of the mass of a baryon. The same difficulties apply here. Nuclear reactions may transform, but do not destroy baryons.
The particles which have been ‘observed’ and which have a ‘mass’ somewhere between that of an electron and a proton all have ‘lifetimes’ measured in microseconds or much less. They are called particles because they leave a trail, footprints, which call attention to something. They are also necessary to make current theories work – just as the aether was necessary for the theory of light propagation in the nineteenth century.