In the physics of elementary particles, as far as I know, there can be three definitions of density. 1. density is the ratio of mass per unit length. 2. density is the ratio of mass per unit area. 3. density is the ratio of mass per unit volume. Quarks and leptons are almost point particles. They have mass but their volumes can be zero therefore their density is almost infinite. Photon has zero rest-mass. It doesn't matter what its volume, its density is zero. So in quantum physics, we seem to be dealing with density of both extreme, zero and infinity. Can we really talk about density in quantum physics?