Dark matter, one of the universe’s most enduring mysteries, is not exactly dark — it’s invisible. Astronomers know the cosmic component has to be there from the force of gravity it exercises, but no one has managed to see or capture it. Now, the European Space Agency has approved Arrakihs, a mission designed to study dark matter, and evaluate current theory surrounding it. There is a possibility that the project could find that our ideas as to how the universe works are incorrect, which would be a historic discovery.
The scientific father of Arrakihs is astronomist Rafael Guzmán, who has spent years perfecting the project at the University of Florida, and is now leading the charge from the University of Cantabria and the Spanish National Research Council’s Institute of Physics. “We wanted this mission to be simple, but for it to have optical quality that pushes the limits of what physics allows,” says the astrophysicist, who was born in Don Benito, Badajoz, 62 years ago.
So far, the result has been a space observatory with four small telescopes, measuring a mere 15 centimeters across. Arrakihs’s two pairs of eyes will contemplate at least 80 galaxies similar in shape and structure to the one humans inhabit, the Milky Way, in order to observe light emitted across different wavelengths from ultraviolet to infrared and including the visible spectrum. “We have broken molds,” summarizes Guzmán. “Instead of going with very complex designs, very large telescopes that would have been much more expensive, we are demonstrating that with an unbeatable optical quality, tested entirely in Spain, it is possible to compete with the most advanced missions.”
The European Space Agency has recently extended definitive approval to the scientific mission, which is classified as type F for “fast,” and is designed so that only a decade will pass between its initial selection, which took place in 2023, and its launch. The Spanish team would like to accelerate that timeline even more, and begin in 2030. This is the first time a scientific space mission has been led from Spain, which will coordinate the participation of Switzerland, Austria, Belgium, Norway, Portugal, and Sweden. Its total budget is $371 million and its primary contractor is Satlantis, a company based in Bilbao.
Turning space observation technology around to point it toward Earth is a part of the company’s origins, explains Guzmán. That’s how its primary service came about, which is detecting gas leaks on oil fields. “Detecting gases like methane follows the same methodology and uses the same technology that astronomers use to detect, for example, hydrogen in galaxies beyond the Local Group,” he explains. The same technology can also be applied to early fire warning. “Our mission didn’t first come up with a scientific use case and then design the camera; instead, it adapted the existing camera to the scientific objectives. This has allowed us to move forward very quickly and led us to being selected as a fast mission,” Guzmán notes.
Arrakihs is an astrophysics mission, but its angle of attack when it comes to the issue of dark matter is almost paleontological. According to current theory, galaxies form throughout millions of years by eating dwarf star systems and galaxies that orbit around them. Those dwarf galaxies are destroyed and leave behind a trail known as stellar streams. In the final stage, the system reaches equilibrium, and this stellar trail is faintly imprinted on the galactic halo — a massive, invisible sphere surrounding the galaxy and containing dark matter, whose gravitational force is essential to the galaxy’s existence. These streams “are a kind of fossil record of the galaxy’s history. Although they are very faint, they are still present. They would allow us to test the cold dark matter prediction regarding galaxy formation, according to which all galaxies like the Milky Way should be full of stellar streams,” says Guzmán.
The theory in question is called ΛCDM, and it is the most-accepted model in describing the universe, which is in its vast majority unknown. Lambda, the tenth letter in the Greek alphabet, represents dark matter, which makes up 68% of the cosmos and is one of the greatest mysteries of physics. CDM stands for cold dark matter, which makes up 27%. Lastly, the only well-known fraction, conventional matter, makes up all visible things and accounts for the remaining 5%.
Stellar streams in our own galaxy have been observed by the ESA’s Gaia mission, and other observatories have detected them in Andromeda. What Arrakihs will now provide is a much larger set of observations, spanning between 80 and 100 galaxies, yielding a statistical measure of the presence of these phenomena, which are closely linked to the mystery of dark matter.
It is possible that the observations will confirm the theory, although it would be exciting if they did not. “The current model predicts the behavior of the universe on a large scale fairly well, but it fails when it comes to the planes of galaxies like the Milky Way [that disk-shaped structure where most stars and planets are located, including our solar system],” explains Guzmán. Theories are valid until an experiment comes along that contradicts them. If that happens, it would bring about “a radical change” that would force us to revise them, the scientist concludes.
After its launch, likely from the European spaceport in French Guiana, Arrakihs will enter Earth’s orbit at a distance of about 500 miles from the surface of the planet. It will have three years of official operation and the possibility of being extended, if everything goes well.
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