What I can't buy into is the idea that one can dump so many gravitons into jump space that they achieve a density that could serve as an energy reserve. "Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is." It's a bit like peeing into the ocean, which is essentially what 90% of humanity does.
If supernovae and black holes and the various scifi anomoly of the week stuff isn't creating enough gravitons to flood jump space, what can our tiny little machines do? Yes, anything the ancients built over millions of years would still be tiny compared to stars and black holes (or we'd actually see them from here). 'Coz galactic core black holes are that mind-bogglingly big, man.
Well Grandfather made jumpspace right? So at some tech/understanding level this stuff is manipulable, even if you have create your own dimension with it's own rules to do it.