What seems to be overlooked in Big Bang cosmology is that gravity, according to Einstein, causes space to contract, so he added the Cosmological Constant, as a force to keep it stable.

What Hubble discovered, with cosmic redshift, is the measure of space between these gravity wells of galaxies expands. The fact is they balance out. The rate space contracts into galaxies is balanced by the measure of expansion between them. Omega=1. The universe as a whole, expanding, is redundant.

This is an effect of the relationship of light to matter, energy to mass, not some mathematical geometry of spacetime.

As for Dark Energy, redshift appears to increase proportional to distance in all directions. So it was assumed, within BBT, the initial "bang" sent everything flying away at nearly the speed of light and this gradually slowed, due to gravity, but what they discovered was the rate drops off fairly rapidly, then levels out. So they needed a further source of energy to explain why it didn't just drop at a constant rate.

Yet if we look at it from our point of view outwards, the rate starts off slowly, then builds, eventually going parabolic. Which would be quite logical, if in fact cosmic redshift is an optical effect, that compounds on itself, going parabolic.