This is a somewhat weird question, but here it goes: What would happen if there were only gravitons? Would some other kind of particle appear in the Universe sooner or later? Would there be any chance for mass to appear in such a Universe (assuming that the graviton is a massless particle)? Could black holes form in the absence of mass (perhaps by some kugelblitz-like mechanism)? Sorry if my question is too crazy / stupid and thank you in advance for your answers.