When you work out the EFEs you use the mass and relative motion of massive objects in space. The formulas have proved out because they have been developed in the laboratory of astronomical measurements meaning that we have a wealth of historic data and we have proved that the same formulas that have accounted for the historical data (back testing) also work to predict. The successful predictions to the 10th decimal and better can be repeated forever and it won’t prove that mass curves space; it will prove that humans are very good at determining all of the factors that account for the movement of objects in space and at performing complex mathematical calculations to apply that knowledge. Humans have shown imagination in attributing the cause of the understood effect to warped spacetime. We don't know how mass curves space ... yet.