The umbrella term for the 68 percent of the universe that we can't detect and know nothing about has been given the umbrella term "dark energy." Like wormholes 30 years ago, it is more MacGuffin than science. You could call it aether or magic or any deity name and be just as valid.
But inference says something, or a variety of somethings, must be causing the universe to expand when gravity says it should contract. So dark energy it is.Except maybe it isn't