My own pet theory is that matter is a combination of superposed electromagnetic waves. Individually they travel at the speed of light, but we see particles when two or more interfere, and the interference fringe moves slower, maybe even stands in one spot. And because it is an interference pattern, it can manifest force lines at a distance, e.g. a magnetic field, and also be converted back into photons and other types of particles if they collide with sufficient energy. This suggests that very short wavelength photons have the ability to break up these interference patterns, and re-assemble or re-configure them to manifest different particles. Thus matter and energy are interconvertible.

I may be right or wrong. I don't know.