The statements that "spacetime is curved by matter" and that "curved spacetime tells matter how to move" summarize GR succinctly. But what exactly is this entity which we label "spacetime" and which gets curved by matter and energy? Think of a test particle in a vacuum - what is the nature of the physical entity which is in direct contact with that particle, the curvature of which is caused by matter and energy? Is it: a) A fluid whose physics we have not yet understood? b) A sea of gravitons (which we hope to observe at the LHC very soon)? c) An entity which lives in dimensions we cannot (yet) observe? d) We dont know - the best description we have is the mathematical model for it - a "Pseudo-Riemannian 4-D manifold with a metric" e) Something else? Thanks for your help with this.