The question, where does the stuff go, I'm guessing of a couple things: 1. Maybe down black holes? 2. Maybe the loss of mass in nuclear fusion reactions? I'm thinking as the universe looses mass the universe expands.
The thought is that given that mass deforms space-time, if mass is "magically" removed from the universe, space-time gets flat. As space-time flattens out, it expands. To illustrate the thought: take a bed sheet, 4 people and a very heavy bowling ball. Put the bowling ball in the middle of the bed sheet, then have the 4 people pull from the 4 corners, the sheet would not be flat as the bowling balls weight would pull the corners close together. Someone walks by and takes the bowling ball away, the bed sheet flattens out and the result is the 4 corners are farther away from each other. Expansion.
Also, to an earlier reply, I was not saying that mass leaves the universe due to expansion. I'm saying mass leaving the universe (or mass being converted to energy) would cause expansion of the space-time continuum because the mass is not there to deform it anymore.