I have wondered about this for some time: Is dark energy really energy and is work actually being done when it causes the Universe to expand? It seems to me that work is being done in moving all the Galaxies further apart than they were, over coming their gravitational pull. For example a lot of work would need to be done to move the earth into an orbit further from the sun. So what is the source of this energy? If it is some kind of stored potential energy then what is its form and where is it stored? I vaguely recall someone saying that the word energy is not a very good word for describing the effect.