Peter Donis and Nugatory taught me a lot about spacetime curvature yesterday, but it has left me with so many questions. It sounds like mass slows down time as it warps spacetime. So, I suppose this means: more mass = more spacetime curvature = less time elapsing. Okay, in addition to that, we know that time "stands still" for the photon. I'm sure there's a more elegant way of putting this, and please let me know. Here's my question: if--this is a big, big if--you could fill the known universe with mass, which, I suppose, would be a complete warping of spacetime, would time be nonexistent, as it is for the photon? I know there's not enough mass and energy to do this, so you could immediately dismiss this course of thought, but I'm trying to get a better understanding of the reach of spacetime curvature. However, if the universe (even in our ideal universe with as much mass as we need) cannot be filled completely with mass (which, again, is complete warping of spacetime), what is it about space or spacetime that prohibits this? Is there some interaction between spacetime and matter that demands that there should always be more spacetime than matter? I think the obvious answer is: matter cannot be created or destroyed, and space is expanding. But I'm just trying to understand that a little better. Thank you for your time.