I just had a radical thought.
It's probably wrong, as it is the product of ignorance. As such nothing that follows should be seen as factual. It is supposition. And again, probably very wrong.
Still, What if the physical volume of spacetime is far larger than it currently appears, the force driving spacetime expansion is the energy that creates vacuum fluctuations entering the true ground state (as more spacetime that has fewer fluctuations), and the currently observed universe's rate of expansion is an illusion?
Shortly after the bang, we have a very excited vacuum, and a volume for the universe that is very constrained. Mass-like fluctuations in the vacuum will occur frequently, even though actual massed particles don't exist yet. Over the volume constrained universe, this creates knots in the energy density of the early universe, by making spacetime "lumpy".
If we presume that the rate of spacetime expansion of this early universe is "just slightly" greater than this gravity like influence from the combined action of the fluctuations that have mass like terms, then spacetime will explode away from the soup, faster than the soup expands. If we say gravitational effects propogate over spacetime at exactly c, then this expansion would be a tiny fractional bit in excess.
the aggregation of this soup toward its barycenter would be arrested by this expansion. The acceleration of the soup toward its baycenter would make the apparent rate of expansion seem very tiny. (Say we are accelerating at 1 plank unit every plank second, toward the net barycenter. We are in spacetime that is inflating at a net rate of 1.000000000.....1 plank units every plank second. The members of our cloud will appear to be moving *away* from the barycenter at the .0000000....1 plank units per plank second, despite actually accellerating toward it.) The actual expansion of spacetime will be considerably greater than the apparent one.
Gravitational attraction falls off on the inverse cube of distance. As the cloud expands (or rather, is pulled apart by expanding spacetime), the rate of accelleration by gravity toward the barycenter diminishes, making the apparent expansion rate increase.
Now, the really odd thought.
If we presume that the driving force behind the expansion is the decay of vacuum energy to its lowest possible state (spacetime with no fluctuations), then rate of expansion will not remain constant, and will slowly degrade over time as it runs out of energy.
This suggests a number of things. First, that the energy density of spacetime (as a whole) is falling off at a greater than geometrical rate in proportion to its volume. Second, that the rate of expansion is actually slowing, as the energy behind the expansion is depleted. And thirdly, all massive objects in the universe are currently travelling with a very large extant of momentum toward the original cloud's barycenter (with some local difflection of vector from uneaven distributions, and interactions with nearby massed objects with local barycenters) that is already a significant fraction of c.
This would seem to explain a good deal.
1) where did all the missing energy go? It's basically empty and flat spacetime surrounding the visible universe like a bubble. It is far bigger than the visible universe.
2) the odd shifts in rates of expansion of the universe over time, when run backwards with regard to star lifecycles, and isotopic concentrations of clouds and star clusters. The universe appears to expand very predictably, slows down for a long time, then suddenly picks up again with great force. The reason suggested: expansion is at first very pernicious, but attractive forces nearly cancel it out, making for slow apparent expansion. The rate of decay in that expansion is not sufficient for attractive forces to overcome. The distances between major massed objects continues to increas, and the rate of accelleration between them diminishes, but slower than the falloff in actual rate of expansion. The two falloff curves (expansion and falloff of attraction) almost join. Expansion continues, but appears very very slowly. Distances between objects continues to increase, reducing attraction so that the curves again seperate, rate of attraction now decays faster than rate of reduction in expansion. Universe starts to fly apart at exponential rate. Voila, we have our bell shaped curve in rate of apparent expansion.
But we also have another sticky mess.
Under this thought experiment, all mass in the universe has been accellerating (counter to expansion), and is now a significant fraction of c in true velocity. This means it is experiencing time dilational effects, in comparison to the actual rate of spacetime expansion, universe-wide. That is to say, the empty spacetime outside our visible universe experiences time significantly "faster" than ours.
But this also explains a curious observation of our current observable universe. The speed of light appears to be changing. (Rather, the speed of light is the same, but the amount of time being experienced is changing.)
If we, as massed particles in motion, are slowing down our rate of hidden accelleration due to the relentless expansion, then our degree of time dilation will also diminish, resulting in the change in measurements. We are slowing down (or rather, not accellerating at the same rate), so we experience more time, and the speed of light appears to slow accordingly.
This means that the actual age of the universe is "waaaaaaaaaaaaaaay" older than what our astrophysicists have determined by measuring star lifecycles, and is likewise, vastly larger in true volume than measurements in our matter dense region would suggest. (By many orders of magnitude.)
This should produce useful math that makes useful predictions that could then be experimentally evaluated.
But again, it is probably wrong in more ways than can easily be counted.