Matter without mass: do we really need the concept of mass?

Einstein's most famous equation -- $E=mc^2$ -- generated a short-circuit between the concepts of mass and energy, which also affects other concepts like matter, radiation, and vacuum. Physics currently has a mixture of classical, relativistic, and quantum concepts of mass, which generates a great deal of confusion and many problems. Clear definitions need to be established if one wants to avoid ghost hunting. In particular, by abandoning the idea of mass and focusing on time and energy, some interesting implications are emerging. There might be an intriguing relationship between the dark energy, as the zero-point energy of the electromagnetic field, and the Cosmic Microwave Background epoch, as its horizon. This might also be the hint of an expanding and rotating universe. An observational test is proposed to verify this hypothesis. I would like to share some reflections on this topic, without claiming any conclusive recipe.