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) ,
- And that post-big-bang expansion was being retarded by existing mass ,
- But since then mass has been lost to the universe 
- Which reduces the (retarding) due to gravity (less mass) 
- Resulting in the "space-time" of the universe expanding faster 
I was never a cosmologist, so please point out which of my assumptions are provably invalid!
 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?
 Major assumption on my part!
So I guess there are two questions here:
A. How confident are we that mass is not being lost in the universe?
B. Would such a mass-loss be able to explain the observed accelerating expansion?