The discovery of this elusive force dates back to 1998 when astronomers found that the expansion of the universe is accelerating. The expansion itself was initially driven by the energy from the Big Bang, but since the Big Bang happened so long ago (13.8 billion years ago, to be exact), this expansion should be slowing down.

To make up for this conundrum, theorists postulated that some mysterious force, dark energy, must exist that acts against gravity and drives matter apart. Cosmological models show that dark energy accounts for 68% of all energy in the cosmos, according to NASA. But astronomers admit that the evidence for its existence is a bit vague.

"[The acceleration of the expansion] of the universe doesn't make sense when you think that there's just gravity there," Isobel Hook, a professor of astrophysics at Lancaster University in the U.K. and a Euclid scientist, told Space.com. "It shouldn't be slowing down. So the fact that we observe it getting faster means that there must be something else. And we just call that thing dark energy because we don't really know at all what it is."