Steve,
I think gravity is not so much a property of mass, as a full spectrum effect of energy coalescing into mass. Quite simply, when we release energy from mass, whether structural order, chemical, atomic, quantum, etc, it naturally occupies more space and the resulting effect is pressure. So what would the opposite effect be? A vacuum. Much of the area supposedly occupied by dark energy, on the outer portions of galaxies, there is a lot of cosmic activity going on and the creation of of increasingly dense and complex mass seems to be the result. E=mc2, then M=E/c2.
Such a full spectrum composite effect would explain why it is so effectively described topologically.
As for dark energy, it is a patch to explain why the ratio of redshift drops off fairly rapidly over the furtherest out/first part of the expansion and then flattens out since. The assumption being that there has to be some natural expansion to explain this flatter, closer decline, not caused by the initial "Bang."
Now if we were to commit cosmological sacrilege and consider what would be observed if redshift was caused by a lensing effect, for one thing, we would be at the center, since we are at the center of our view of the universe, which would go to our previous discussion of why using relativity to explain this anthropocentric effect overlooks the fact that to be relativistic expansion, the clock rate would have to increase as well, in order to remain constant.
Another effect would be that it would compound on itself and thus go parabolic. Creating the impression that the further away the source, the faster it is receding. Which is exactly what we do see. It is not that redshift drops off, then flattens out since the Big Bang, but that it increases exponentially the further the source is and it is that upward curve in the rate that is being observed.
No, we and I don't have all the math and the details, but finding those details might be a far more productive and feasible endeavor than exploring for other universes.
We accept that gravity can be modeled as acceleration and call it the equivalence principle. Could there be an opposing equivalence principle, by which the expansion of light looks like recession of the source? The fact is that this is science and alternative view points should be open to examination, not simply dismissed because too many people think differently.
Regards,
John M