Thank you, you're a fantastic writer and clearly have some expertise. I am not on your level but do spend a decent amount of time trying to deepen my understanding of physics.
This got me thinking, would one way to explain expansion possibly be gravity is slowly getting stronger on shorter distances, or that the fabric of spacetime itself is not perfectly rigid, not only in the "depth" component like the classic trampoline analogy, but also in the "length/width" component? Galaxy filaments are thinning, so if you think of the center of a supervoid surrounded by filaments on all sides, that void is being stretched apart in every direction, at some level that is so fundamental that it "creates more space". Then again, everything everywhere is surrounded by filaments and all space is being pulled apart by the same reasoning, but if there is anisotropic mass close enough, this overrides the creation of new space.
The tl;dr is that the implosion of early dense baryon clouds created shockwaves which threw most of the matter (including dark matter) out of the regions that later became cosmic voids.
Few voids are outright surrounded by denser parts of the cosmic web, and there are many "invasions" of dense filamentary structures into large voids. Additionally, at much larger scales the difference between relatively empty space and relatively full space blurs away (WiggleZ Dark Energy Survey, SDSS-BOSS) preserving our ability to work with the spatially homogeneous and isotropic distribution of matter in the concordance cosmology. Thus the matter an be modelled as a uniform "dust" whose individual motes move only with the expansion, and even as a set of perfect fluids which carry attributes such as pressure, density, velocity of sound, and effective equation of state.
It is pretty reasonable to expect that ongoing detailed sky surveys will find small numbers of galaxies and/or quasars in most voids, as any reasonably dense concentrations of matter left behind from the BAO would tend to collapse gravitationally. The orbits in such in-void structures will be important observational targets. One should expect that these galaxies (and the subsystems within them) do not expand with the cosmos any more than our galaxy or our solar system, and that their peculiar against the cosmological comoving coordinates is low, like galaxies clearly outside voids.
This got me thinking, would one way to explain expansion possibly be gravity is slowly getting stronger on shorter distances, or that the fabric of spacetime itself is not perfectly rigid, not only in the "depth" component like the classic trampoline analogy, but also in the "length/width" component? Galaxy filaments are thinning, so if you think of the center of a supervoid surrounded by filaments on all sides, that void is being stretched apart in every direction, at some level that is so fundamental that it "creates more space". Then again, everything everywhere is surrounded by filaments and all space is being pulled apart by the same reasoning, but if there is anisotropic mass close enough, this overrides the creation of new space.