What on Earth is dark energy?

Something in the universe is pushing everything away from everything else, and it's getting faster and faster. But scientists don't know what it is.

Dark energy is the unknown mechanism behind the ever-accelerating expansion of the universe.
Deutsche Welle

Despite the leaps and bounds science has made over the course of the past centuries in increasing our understanding of the world around us, there’s still a lot we don’t know about how everything fits together.

Scientists believe that something called dark energy exists all around us, composing almost 70 per cent of the universe. But like its mysterious counterpart dark matter, which makes up the bulk of the remaining 30 per cent, they still haven’t actually found it.

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Where does the idea of dark energy come from?

By the early 1990s, scientists had made great progress in our scientific understanding of how our universe came to be, largely through the framework of the Big Bang Theory.

The theory, first proposed in the 1930s, said the universe started as a super-hot, super-condensed tiny point that quickly started to expand and cool.

Scientists believed gravity would eventually slow that expansion. But research conducted near the turn of the century showed this wasn't the case — in experiments for work that would eventually win the Nobel Prize, scientists Adam Riess, Saul Perlmutter and Brian Schmidt realised the expansion of the universe was not slowing.

Much to the contrary, in fact: The researchers found that the universe was not only continuing to expand — it was doing so in an accelerated way. This means that faraway galaxies are not only drifting away from us, but they’re doing it faster and faster as time passes. Scary, huh?

Further observations confirmed the scientists' findings, proving it wasn’t a fluke — the universe really is expanding at an ever-quicker rate. This knowledge came as a huge, puzzling revelation to scientists and represented a sort of paradigm shift.

Gravity, scientists conceded, was no longer the dominant force in the greater scales of the universe. Something else is lurking in there. Scientists started calling this mysterious accelerator "dark energy."

Saul Perlmutter (left), Brian Schmidt (right) and Adam Reiss (middle) shared the 2011 Nobel Prize in Physics.
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The accelerated expansion of the universe

Scientists describe dark energy as a kind of opposite to gravity. It pushes things away from each other.

"Huh?" you might be thinking. "Why isn’t everything flying away then? How are objects able to stay in place?"

Gravity is why. On the smaller scale, for gravity-bound objects like your smartphone, planets around the sun or the stars in our galaxy, gravity beats dark energy. But for very distant galaxies, dark energy wins.

A good way to visualise this is by taking a slightly inflated balloon and drawing some dots on it with a marker. Blow it up and, as it inflates, you can see all the dots separate from each other. Replace the dots for distant galaxies and that's kind of like what happens in our universe — the air represents the dark energy, gradually "inflating" the universe and pushing objects apart ever so slightly, stretching the very fabric of spacetime.

And in case you were wondering: Dark energy represents something different from dark matter. The latter is a yet invisible type of matter that has a gravitational effect, meaning it attracts things, and helps galaxies keep their shape and rotation. Contrary to dark matter, dark energy pushes things away from each other.

The farther the galaxy, the faster it seems to be moving away from us. This James Webb Telescope image shows some of the farthest galaxies ever observed.
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What is the fate of our universe?

Although current evidence points to this accelerated expansion going on forever, could it change in the future? Could it slow down or increase? Will it just stop or start shrinking?

Those fundamental questions haven't been answered yet, because scientists haven't figured out exactly what dark energy is. Once we can answer these queries, we’ll know a lot more about the fate of the universe. 

Researchers are looking to answer these crucial questions through systems like the European Space Agency’s Euclid telescope. It’s aimed at improving our measurements of the rate of expansion of the universe — and thereby teaching us more about dark energy.

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The Euclid space telescope, launched in July 2023, is tasked with creating the most detailed map of the universe
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So…any idea what dark energy could be?

There are some candidate ideas, but they are still a long way off.

One explanation is that dark energy is a type of energy that is simply part of empty space, an intrinsic property of spacetime. As there's more and more space, the effect increases, accelerating the expansion.

Another possibility is that just as much as Einstein's general relativity was an improvement over Newton's laws of motion and gravity, perhaps there's a better explanation — a new gravity theory that better explains the evolution of the universe at a very large scale and its accelerating expansion while also explaining the motion of planets as Einstein's theory does.

A few theories have tried to do this but, so far, none have been successful.

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