Based on current models of cosmology, dark energy makes up around 68 percent of the Universe's total energy.
Why we need dark energy
The expanding state of the Universe was understood for the better part of the 20th century, but most physicists had assumed the infinite reach of gravity would eventually bring galaxies together again at some point in the future.
This didn't turn out to be the case.
Observations of distant supernovae in the late 1990s found these far-off exploding stars were fainter than anticipated, meaning they were probably farther away than we first thought.
When these data were combined with existing cosmic models, the measurements suggested the expansion of space had sped up over time.
Here's where dark energy comes in: something powerful would have to be responsible for adding an extra push to the Universe's growth. Yet no such energy known to physics can account for the repulsion, leading physicists to refer to the mystery as 'dark' energy.
Current hypotheses say that dark energy emerges from empty space, powerful enough to overcome the gravity that pulls clusters of galaxies together, all without ripping them apart from within.
There have also been attempts to get rid of dark energy in our current cosmological model, but so far we don't have enough evidence to ditch it.