Expanding space; does it have momentum?

With the virtual particles of space equating to zero, the universe is comprised mostly of empty space, the average density of matter being somewhere between 10^-30 g / cm^3 and 5 x 10^-30 g / cm^3, or if you prefer, very low indeed. So, what is it that continually causes non-mass 'space' in the universe to expand; to stretch out between matter, and even to stretch out light into longer wavelengths? What could dark energy act upon to expand space at accelerating rate if space has no mass and by definition no momentum?