This weekend, the European Space Agency launched Euclid, a next-generation space telescope on a mission to observe billions of galaxies and help solve some of the universe’s deepest mysteries. Carried to low Earth orbit on a SpaceX rocket, the telescope will spend a couple of weeks deploying its sunshield to reach operating temperature, before travelling to a point 1.5 million kilometres from Earth (AKA the second Lagrange point, or L2) at the end of the month. There, it will spend the next six years monitoring one third of the sky.

Euclid’s task? To create a massive, 3D map of the universe, which will help scientists understand its overarching structure, and how it was formed and expanded over the course of cosmic history. Hopefully, this will include some revelations about the nature of two of the universe’s most enigmatic elements: dark matter and dark energy.

What are dark matter and dark energy? Well, not even Earth’s greatest scientists can say for sure. This is quite a shocking fact, considering current estimates suggest that they collectively make up around 95 per cent of the universe – roughly 27 per cent dark matter, and 68 per cent dark energy. (In case you can’t be bothered doing the maths, this means that everything we see on Earth, all the stars and galaxies in the night sky, and all “normal” matter beyond that, makes up a meagre five per cent.) What’s more, dark matter and dark energy are linked to the accelerating expansion of the whole universe, which has perplexed scientists since it was discovered in the late 90s.

Even though the dark universe takes up so much space, it’s proven very elusive (the clue’s in the name) and currently we only have theories and speculation about what it actually is. So, let’s start with dark matter. The favourite theory to explain dark matter is that it’s an undiscovered type of particle that interacts too weakly with normal matter to be observed by conventional means.

“Large, but dark, objects are mostly ruled out because we would see their gravitational effect on visible objects or light from stars and galaxies,” explains Andy Taylor, a professor of astrophysics at the University of Edinburgh. (The black hole at the centre of the Milky Way, for example, is observable due to its effects on surrounding matter.) However, there are many hypothetical particles that could explain dark matter. Taylor adds: “There had been some hope that the Large Hadron Collider might be able to create some of the proposed particles, but so far we’ve seen nothing. This leaves open a large number of possibilities.”

Unfortunately, dark energy is even more mysterious, with lots of competing ideas about its existence. (As Taylor notes: “Theorists have been busy!”) According to Tessa Baker, a cosmologist at Queen Mary University of London, we’re not even sure if dark energy is an actual substance, or simply a misunderstanding of the laws of gravity. Effectively, it’s just a “placeholder name” for whatever it is that’s causing our universe to expand at an accelerating rate. It could be tangled up with how quantum mechanics operate in the vacuum of space, or it could be a new “fundamental field in the universe”. Right now, we simply don’t know.

Dark matter and dark energy have to exist in some form, though, if we’re to explain the behaviour of the five per cent of the universe we can observe. “If we don’t include them in our theories, we can’t get anything to match what we see, so unfortunately they are a critical component of the universe,” Taylor explains. “We don’t know why they are ‘dark’, but if we want to explain what we see in the universe, they must be there. Hopefully, a deeper understanding of their properties will help us to understand why they are dark.”

So, back to Euclid. How will the space telescope help us to pin down the properties of the dark universe, exactly? On a basic level, “Euclid will study the positions and shapes of millions of galaxies,” says Baker. These galaxies are sometimes called the “cosmic web” – a “spiderweb-like or sponge-like structure” with an overarching sense of order. “The precise details of this cosmic web (the sizes of the holes in the web, for example) are finely controlled by gravity,” she explains. As a result, any changes to the laws of gravity will result in changes to the web’s appearance, which we can compute back on Earth. This should help rule out some theories about the dark universe and – “if we’re lucky” – direct us toward others that seem correct.

“Euclid will study the positions and shapes of millions of galaxies... sometimes called the ‘cosmic web’” – Tessa Baker

One of the main techniques that will be used to gather data is gravitational lensing, which involves studying the distortion of light as it travels across the universe. “We see distant galaxies because light has travelled for vast distances and millions of years across the universe to reach our telescopes/eyes,” says Baker. “We think of light rays as travelling in perfectly straight lines, but actually this isn’t always true. If there’s a really massive object nearby – like a cluster of 100 galaxies – the gravity is strong enough to change the paths of light rays, causing them to bend and warp.” By comparing the distorted image we receive to the actual object in space, we can learn something about the mass that warped the light rays en route to our eyeballs. As we already established, the vast majority of the mass in the universe is in the form of dark matter, so this should tell us a lot about how it’s distributed among galaxies and galaxy clusters.

For the most part, however, the distortion of individual light rays is so small that it isn’t particularly useful to researchers. Euclid is set to change that, due to the sheer quantity of the universe that it can observe in precise detail, using both an infrared and an optical camera. “We have to measure millions of galaxies to build up enough statistical power to make the measurements,” says Baker. “This is precisely one area where Euclid will amass new and powerful data.”

Euclid isn’t the only tool we’re using to investigate the dark universe, though. Back on Earth, ground-based telescopes provide complementary data, with the largest – the Vera Rubin Telescope – set to start operating within the next few years. “It will be exciting to see how its results compare to Euclid’s,” says Taylor. Then, there are devices called gravitational wave detectors, which detect “ripples in the fabric of spacetime” caused by the merging of black holes. These are currently based on Earth, but will eventually be sent into space as well, creating even more opportunities to learn about dark matter and dark energy.

If you’ve come this far, you might be wondering: what’s the point? Is it really worth investing so much into understanding the dark universe? What does it mean for us, back on planet Earth? Well, to the best of our knowledge, there are no known effects of dark matter or dark energy on human beings or any other living things (at least directly). Baker draws a comparison with neutrinos, a fundamental particle that is constantly emitted by everything from the sun to a banana. “There are thousands of them streaming through your body right now!” she says. “But, no interactions, no problem. So it’s possible for particles to be around us and not harm us.”

“Studying the dark universe is absolutely crucial, fundamental science regarding the building blocks of the universe” – Tessa Baker

That being said, we might not even be here if it wasn’t for the effects of the dark universe. “This is absolutely crucial, fundamental science regarding the building blocks of the universe,” Baker adds. “If dark energy is found to be a new fundamental field, and dark matter a new fundamental particle, then we’ll have to pretty much rewrite all physics textbooks from scratch! Plus, what kind of scientists are we if we only understand five per cent of the universe?”

Taylor agrees that figuring out the dark universe is a vital and far-reaching scientific effort. According to our current understanding, “dark matter provide[s] an environment where galaxies and stars and planets can form,” he says. “Without dark matter it’s possible the universe wouldn’t form these, at least not as we see them, and that might make life less likely in our universe.” Proving or disproving the theory that dark energy is caused by “quantum fluctuations” in the vacuum of space could also have knock-on implications for quantum mechanics, which is “perhaps our most successful theory” in terms of explaining the fundamentals of the universe. Basically, a significant discovery about the dark universe could rewrite reality as we know it.

“Perhaps more poetically, humankind has built lots of different models of the universe and our place in it for millennia,” Baker concludes. Obviously, many of these models have been very wrong – after all, we believed that the Earth lay at the centre of the universe for more than 1,000 years. However, “we are gradually inching closer and closer to a final, true model that won’t change”. If this is true, understanding dark matter and dark energy will be a crucial part of the journey.

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