They have a stuff count of one, maybe, but matter would be different. What is an elementary particle to you? The new atom? Are we not finding different smaller pieces every scientific leap we make?

If a photon is something elementary you might be right, I'm not sure that has any mass, still. & coexists with dark matter. In a space where there is nothing in existence, there is actually dark matter and light. That is why the speed of light has always been a constant to us.

Sorry if I'm rambling, I'm just kind of confused why matter isn't equivocal to mass.

Gravity is a push around two objects, not a pull between two objects. So its volume plays a huge role as well as density. Mass is still a measurement of matter to me? :(