Dark energy is an unknown force hypothesised to be responsible for the accelerated expansion of the Universe – an effect that we've observed, but haven't been able to fully explain.
Based on current models of cosmology, dark energy makes up around 68 percent of the Universe's total energy.
Why do we think dark energy exists?
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.
It turns out that this isn't case.
Observations of distant supernovae in the late 1990s found the exploding stars were fainter than anticipated, meaning it was likely they were also 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 propose dark energy might emerge from the bubbling of empty space, a small effect that is also widespread, making it powerful enough to drag apart clusters of galaxies 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.
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