[PatrickHenry]

... the energy content of a vacuum is non-zero with a negative pressure, in other words, dark energy ...

I donno. More energy is actually more matter/energy, which automatically means more gravity. And if gravity has a minus sign (as it must to cancel out matter/energy and result in a net zero for the universe), then this new dark energy stuff must be generating more gravity, which automatically cancels it out and we'd still have a net energy component of zero for the universe. Makes sense? We need Physicist to come to our aid.

[VadeRetro]

I'm not sure. "Dark energy" isn't gravity and acts rather opposite. The expansion creates more "vacuum," which has non-zero energy. The gravity of the virtual particle stuff is contractile and tends to cancel the energy, but the dark energy is oppositely directed and so tends to create an increase in net energy with expansion?

I was out of my depth when I was looking for my boots this morning, so I'll wait for Phys to come and decode for the laymen.