You should look into the cashier effect. It talks about how even in empty space, it seems there's some sort of standing energy in that vacuum state, almost like the fabric of space-time itself contains energy.

I like this idea that's in the TV show Stargate Atlantis for their super duper high energy power source that runs all their sci-fi equipment. Basically, they make an artificial instance of subspace or of another dimension, then extract the energy in that little isolated dimensional space, and continue to draw that huge amount of energy out until this artificial region of another dimension collapses due to entropy and their pulling energy from this alternative space. They'd basically be pulling in free energy from another space-time into our own to then use. Of course I imagine even if tech is hidden from us, we're still a very long ways away from being able to create isolated extra dimensional space within our own universe that we could contain and exploit for energy without using more energy than it would take to create such an instance of extra dimensional space.. Anyway, Fun show with some fun concepts in it.