It is postulated (and widely accepted) that even a total vacuum can have a finite energy density: there is the spontaneous creation and destruction of particle - antiparticle pairs. This zero-point energy concept is not without its problems because it turns out that the total energy in the universe would be huge as a result, and this is even after the equations have been "adjusted" to remove the infinity value. The theory is thought to have been possibly verified by the existence of the Casimir effect but actually there are also other explanations for this effect. (Simply put, this is were two precisely machined plates experience an attractive force when brought very close together - there being insufficient space to permit the creation of particle - antiparticle pairs and thereby resulting in there being more pressure pushing them together than holding them apart).
Although featured in science fiction as a potential source of energy, it is not known how that such energy could ever be made use of. To make use of an energy source there needs to be a lower energy state than the source. By definition, there is not thought to be one in this case.