re: Relative rate of temporal progression
I think these images are kinda explanatory:
Matter in the early universe was essentially a big ball of hot plasma. But it wasn't uniform, so if we go further back in time we would see greater relativistic discontinuities between different regions of space. Particles in less dense regions would travel fast, while particles in the denser regions would appear to slow down.
All things relative, how do we know that our mass-filled region of space isn't collapsing, and merely giving the impression of expanding voids 'out there' ? If there is so much dark matter, then why wouldn't this be reasonable?