The search for a physical “Theory of Everything”, a model in which gravity is described in terms that are mathematically (ideationally) consistent with the other fundamental forces — the electromagnetic, strong nuclear and weak nuclear forces — is an attempt to unify descriptions, not only of different scales, but also of a different order. This is because the other fundamental forces, as construed by Quantum Theory, describe relations between particles of matter, as effected through an exchange of particles, whereas gravity, on the other hand, as construed by General Relativity, describes a relation between matter and its frame of reference, space-time[1], which then has consequences for other particles of matter.[2] The attempt to describe gravity in Quantum physical terms, as effected through an exchange of particles — hypothetical gravitons — is an attempt to model gravity as a direct relation between particles.
Footnotes:
[1] Physicists routinely conceptualise the frame of reference, space-time, as if it were material — as, for example, when the term ‘fabric’ of space-time is used.
[2] The fact that both photons and the gravitational effects of matter travel at the speed of light suggests that this speed is a limit of the frame of reference, space-time.