dark energy is a load of old cobblers. It was invented to fudge a huge flaw in a model for the big bang which was based rigidly on modern physics. It is a figment of deluded scientists imaginations!
Harnessing it could involve combusting a certain type of organic grey matter.
Dark energy serves as an attempt to explain an observation that the expansion of the universe is speeding up. The ideas surrounding it are quite tentative - Einstein's cosmological constant - energy of the vacuum (a quantum effect). There is an alternative model called quintesscence. As you can see, it is all quite speculative.