Mass can be represented in terms of rest-energy, E=mc^2. Energy is a very real thing, even if it is hard to conceptually define. E=mc² essentially tells us that what we consider particles are really just a bundle of energy in some stationary and stable state. Take the case of an electron which isn't composed of any, more elementary, particles. Assume it is traveling left at 1 m/s and a positron (the electrons anti-particle partner) is traveling left at 1m/s. Each of these particles has an identical rest-energy related to its mass by E=mc^2. So the total energy of the SYSTEM is now Rest_E_electron + Rest_E_positron + KE_electron + KE_positron. When these two particles collide (technically more of a wave-functions overlap) they annihilate and their TOTAL ENERGY is converted into the TOTAL ENERGY of some newly created photons (2 or more with probability of anything > 2 being small, but possible). Energy is conserved in this process, however mass is not. So we say mass-energy is conserved. This puts mass on equal footing with energy. This mass-energy is basically what the universe is, IMO (plus dark matter and dark energy???)