It's hypothetical, but less so than a lot of the things theoretical physicists consider. Any quantum theory which reduces to general relativity in the classica

It's hypothetical, but less so than a lot of the things theoretical physicists consider. Any quantum theory which reduces to general relativity in the classical limit will look like a theory of gravitons when quantum effects are small. This is just how the math works out in effective quantum field theories involving massless spin-2 particles.