Why is that general relativity does not define what space is. It just says when there is a stress-energy tensor in space it creates an einstein curvature and to mathematically balance it adds the cosmological constant. Space has to be defined for the equation to make complete sense given the fact that space is expanding. So how is it that something we cannot see is expanding and yet the gravitational curvature is still constant on earth(even though space is stretching out). Does that make you wonder whether Einstein got the equations backward.