The conflict between the two cosmologies derives from the inflexibility of Einstein's theory of general relativity, which shows how gravity is formed by the interaction of matter, space, and time. Physicist John Wheeler famously summed up the theory's essence as "Matter tells space how to curve; space tells matter how to move." However, in order to build a workable cosmological model, all of the terms on both sides of Einstein's equations must be balanced: on one side, matter (i.e., all the things that warp time and space); on the other, the curvature of the universe and the speed at which space-time is expanding. In short, a model requires a particular amount of matter in order to produce particular curvatures and expansion rates.