If expansion is motion, and space is expanding, and we consider space as void, how can it then move? Don't we have to consider space as "something" if it moves? If we consider space as "something", doesn't that launch us back to ether theories?
If there were nothing between the grains I believe the grains would be touching each other. Space-time is something. Try thinking of time as a one dimensional object. Now you can see that our universe is just a one dimensional duration, and as it expands, we with our view from the inside looking out see this over all motion as space expanding. What we call reality is made up of all the motion in all its different combinations that are confined inside this one duration we call our visible universe, and yes it is filled with space-time.If there is nothing between those grains, what meaning is there in saying the "nothingness" has a velocity? After all, we can only measure things.