In just a few weeks, a remarkable European probe will be blasted into space in a bid to explore the dark side of the cosmos. The €1bn (£850m) Euclid mission will investigate the universe’s two most baffling components: dark energy and dark matter. The former is the name given to a mysterious force that was shown – in 1998 – to be accelerating the expansion of the universe, while the latter is a form of matter thought to pervade the cosmos, provide the universe with 80% of its mass, and act as a cosmic glue that holds galaxies together. Both dark energy and dark matter are invisible and astronomers have only been able to infer their existence by measuring their influence on the behaviour of stars and galaxies. “We cannot say we understand the universe if the nature of these dark components remains a mystery,” said astrophysicist Prof Andy Taylor of Edinburgh University. “That is why Euclid is so important.” Taylor added that UK scientists had played a key role in designing and building the probe. For example, one of its two main instruments, the craft’s Vis imager, was mostly built in the UK. “We thought what would be the biggest, most fundamentally important project we could do?” Taylor said. “The answer was Euclid, which has now been designed, built and is ready for launch.” Euclid was intended to be launched last year on a Russian Soyuz rocket. However, after the invasion of Ukraine, the European Space Agency ended its cooperation with the Russian space agency, Roscosmos, and instead signed a deal to use a Falcon 9 rocket from Elon Musk’s SpaceX company. Scheduled for launch on 1 July, Euclid will take a month to cross the solar system to its destination 150 million km from Earth at a position known as the second Lagrange point. Here the craft will be able to peer out into deep space with the sun, Earth and moon behind it. The 2-tonne spacecraft will then begin its survey of the heavens. “Euclid has the resolving power of the Hubble space telescope but will be able to survey a third of the night sky at the same time, so it will give us an incredibly detailed map of the heavens,” the astronomer Stephen Wilkins, of Sussex University, said. Such precision will be crucial in uncovering the secrets of dark matter, which cannot be seen directly because it is most probably made up of particles that do not emit, reflect or absorb light, according to scientists. To get around this problem, Euclid will exploit a phenomenon known as gravitational lensing. This will involve taking millions of images of galaxies. In some cases, light from these distant bodies will pass through dark matter on its journey towards Earth. When that happens, its gravitational field will stretch and deflect the path of the light. This is gravitational lensing and the distorted images it produces will provide key insights into the nature of the dark matter that is triggering them. “Gravitational lensing produced by dark matter will tell us a great deal about what it is made of,” said Prof Mathilde Jouzac of Durham University. “It may be that dark matter is made up of light particles. If so, they will produce one kind of lensing. On the other hand, if dark matter is made of very large particles, that will produce a different set of lensing. This information will then help direct the search for dark matter particles on Earth.” Then there is dark energy. “We will use Euclid to measure it in a different way,” Wilkins added. “We will peer further into the universe and further back in time and study how big it looks at different periods. In that way we can actually work out how the size of our universe is changing over time and understand when changes in its expansion rate occur.” The end result of this work will be the creation of a history of how the universe has expanded over the past 10 billion years, a chronology that should reveal secrets that should help unravel the true nature of dark energy. In this way, Euclid – named after the ancient Greek mathematician who is considered to be the father of geometry – will uncover the secrets of the dark universe, it is hoped. “The point of Euclid is really to get the data that will allow us to start discriminating between which of the different ideas we have about the dark universe,” said Taylor. “Hopefully that will help us understand what fundamentally is really going on in the cosmos around us.”