The photons that make up the condensate have no rest mass, which is why they travel at the speed of light whenever they have a non-zero amount of kinetic energy

The photons that make up the condensate have no rest mass, which is why they travel at the speed of light whenever they have a non-zero amount of kinetic energy. Anything that has any form of energy has mass, since those are just two different words for the same property. Because of this, the photons can be called "massless" (i.e. no rest mass) but still have mass, and the condensate could in theory have mass without any of its constituents having mass. The photons have mass from their individual kinetic energies, and the condensate could have mass beyond that from any potential energy required to get it into that state. Although, if the condensate is stable in any sense, it probably has less mass than the sum of the energies of its photons, with the difference being the loss of potential energy it gains from being stable. As a comparison, atomic nuclei are composed of particles with non-zero rest mass, and they tend to have less mass than the sum of their components because of their stability. The difference between the mass of the nuclei and its components is how much energy you have to dump into the nuclei to split it (plus enough kinetic energy so that they fly away before they reform). As to the question of what constitutes "matter"... I'm not sure that's actually defined in a way that's 100% free of contention. My personal opinion would by any configuration of particles that's stable. This new stuff could qualify.