Dark Energy is greatly beneficial to demonic entities and makes them far more powerful than normal.
Dark Energy can also grant transformations in some case as it allowed the demon Towa to take on her race's Darkness form, and allowed the Spice Boys to undergo the Makyan Gigantification transformation. A Demon God can use Dark Energy to take on the Giant Demon God form. Dark Energy can also power up evil beings, and when utilized by an evil entity who is not a member of the Demon Realm race, it can allow the user to transform into a King of Destruction.
This energy normally only exists within the Demon Realm and Makyo Star, however Towa and Demigra managed to develop Dark Energy that could exist outside of these spaces, and used it to try and corrupt multiple worlds.