How confident are we that mass is not being lost in the universe? After reading about the latest super-massive black hole in Nature 518, 512–515 (26 February 2015), I couldn't help but wonder if the accelerating expansion is a result of mass being lost.
My reasoning is as follows:


*

*If the early universe had a particular (greater) mass than at present,

*Then "space-time" could have had a "momentum" (determined by that mass) [1],

*And that post-big-bang expansion was being retarded by existing mass [2],

*But since then mass has been lost to the universe [3]

*Which reduces the (retarding) due to gravity (less mass) [2]

*Resulting in the "space-time" of the universe expanding faster [4]


I was never a cosmologist, so please point out which of my assumptions are provably invalid!
[1] Does spacetime have a "mass" value? or What is "Spacetime" made out of?
[2] I'm struggling to remember my undergrad physics - would two particles each with an initial velocity moving away from each other in a gravitational field (relatively) speed up if the gravitational field is reduced?
[3] Major assumption on my part!
[4] Maybe!
So I guess there are two questions here: 
A. How confident are we that mass is not being lost in the universe?
and
B. Would such a mass-loss be able to explain the observed accelerating expansion?
 A: You ask I'm struggling to remember my undergrad physics - would two particles each with an initial velocity moving away from each other in a gravitational field (relatively) speed up if the gravitational field is reduced?
To answer this part of your question,  gravity only works in one way, pulling things together, never allowing them to move away from each other. If the gravity field was reduced, then they would still come together, just more slowly than before
A: 
  
*
  
*>> How confident are we that mass is not being lost in the universe? << *
  

Mass (energy) can be lost in principle: if you convert mass to radiation (which you can, because mass and energy are equivalent), the radiation density dilutes with the growing scale factor to the 4th power because of the redshift, while mass density only dilutes with the scale factor to the third power (because volume is lenght³). So while the total mass provided by matter stays the same even when it thins out while the universe expands, the energy provided by radiation shrinks because the photons do not only get spread out like normal matter but also get their wavelengths increased and therefore their frequency reduced. Because not only mass is equivalent to energy, but also energy to frequency, energy (and therefore mass if you wanted to convert it back later) can be lost.
