Sorry, I was being brief. I usually say "anything with a non-zero rest mass-energy" for anything for a photon, photons never being at rest. Briefly, some had thought that neutrinos might be little more than floating carriers of lepton number. Later that was amended to merely "massless" (in the sense that they would at least be like photons). I've never been in that crowd. My thoughts are that mass-energy is the coin of existence and gravitation the inevitable consequence, but I do not speak for everyone, just trying to give a kind of non-exhaustive overview of the history to respond to the for part.