The Universe is a jaw-droppingly beautiful place. It contains stunning planets sporting surfaces painted with vivid brushstrokes, dying and dead stars that light up the cosmos with rainbows of different colours, and galaxies that pirouette around one another, pulling long and intricate strands far out into space.
But it’s what we can’t see that’s far more important. There would be no structure in the Universe without the scaffolding upon which entire galaxies are built.
The stars, gas and dust we see in galaxies is just a light dusting of ordinary matter resting upon an invisible frame of dark matter. In a galaxy such as the Milky Way, this dark matter outnumbers ordinary matter by almost six to one.
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“Dark matter is the backbone of galaxy formation,” says Dr Andreea Font, an astrophysicist at Liverpool John Moores University. And without galaxies there would be no stars; without stars there would be no planets; and without planets there would be no life to wonder about the nature of dark matter.
The evidence for the existence of dark matter is wide and varied; stars on the edges of galaxies would fly off into space without the additional gravitational pull provided by dark matter, for example. Yet despite being so pervasive and important, we still don’t know what dark matter is made of.
Stars that look like galaxies
For decades, the leading contender for the thing that comprises dark matter has been weakly interacting massive particles (WIMPs). They were first proposed by physicists as way of solving problems in particle physics.
A theory called supersymmetry suggests that every ordinary particle has a heavier companion. It turned out that the lightest of these supersymmetric particles would behave in just the same way as dark matter.
Many searches have taken place for WIMPs on Earth, but if WIMPs clumped together to form star-like objects in the early Universe, we could look for them in space too.
“WIMPs can’t actually form stars themselves,” says Font. “Instead these early ‘stars’ could have had lots of dark matter inside them.” In the modern Universe, stars are powered by nuclear fusion, where hydrogen is churned into helium and releases energy in the process. There would be no fusion in these so-called WIMP stars, but they would still release heat due to the interaction of all the dark matter particles.
If such stars existed, it might also solve another problem. Astronomers have long been perplexed by the presence of large black holes in the early Universe. These black holes seem to have appeared too quickly to result from the deaths of the first ordinary stars, but it’s possible they might have formed from the deaths of WIMP stars.
During their lives, WIMP stars would be gargantuan compared to ordinary stars. They could be a million times more massive than the Sun and up to a billion times more luminous (in terms of the electromagnetic energy they release, which can be observed in infrared light).
Small galaxies also contain billions of Sun-like stars, so a dark star’s luminosity could match that of an entire galaxy. And that raises an intriguing proposition: some of the earliest galaxies astronomers have spotted might not be galaxies at all? Could they be supermassive WIMP ‘stars’ instead?
Astronomers Cosmin Ilie, Jillian Paulin and Katherine Freese certainly think that it’s a possibility. In 2023 they published the results of their infrared observations of the early Universe using the James Webb Space Telescope (JWST). They uncovered three objects that, at first glance, could be mistaken for galaxies, but may turn out to be dark stars.
“It’s hard to tell the difference from imaging alone,” says Font. That’s because the objects in question are over 10 billion light-years away and each one covers an area of sky around 3,000 times smaller than Earth’s Moon when it’s full. But there is a way we might be able to determine whether the objects are: spectroscopy.
Missing colours
Spectroscopy is one of the most powerful weapons in an astronomer’s arsenal. The technique works by taking light from a distant object and splitting it into its constituent colours to produce a spectrum, similar to the way a raindrop splits light to form a rainbow.
Hidden among the colours are dark bands known as absorption lines. Absorption lines are simply missing colours – gaps in the spectrum left behind after different chemical elements in the distant object swallowed that part of the light.
Absorption lines give astronomers an inventory of the chemical elements that make up the distant object. According to the theory, WIMP stars should contain a particular type of helium that ordinary stars don't. In other words, there'll be an extra absorption line in the spectrum that won't appear in the spectrums produced by ordinary stars.
JWST can check for this line, but it would take months of the telescope’s time to make the necessary measurements. Given how much demand and competition there is for JWST observations, it’s unlikely such time would be given to exploring a proposition which, at this point, is a long shot.
Part of the problem is that WIMPs are fast falling from favour. “We’re scraping the bottom of the barrel,” Font says. Scientists have spent the last few decades building a variety of experiments to search for WIMPs.
They’ve buried detectors in abandoned mines, lowered them beneath the Antarctic ice and even strapped them to the International Space Station.
A lot of time, effort and money has been spent on the search for WIMPs, but so far, there isn’t a single WIMP to show for it. That’s despite dark matter supposedly being all around us all the time – a billion dark matter particles should be passing through your body each and every second.
The fruitless search for WIMPs has led scientists to turn their attention elsewhere and interest is growing in the various alternatives, with axions being one of the most promising.
The axion was first proposed in the 1970s, not as a candidate for dark matter, but in an attempt to solve another problem in particle physics regarding antimatter. All of the ordinary particles that you’re made of, such as protons and electrons, have a corresponding antiparticle. Usually, there’s a small difference in the way particles and antiparticles behave.
When it comes to the strong force (the force that binds atomic nuclei together), however, there doesn’t appear to be a difference. The influence of the axion was suggested as a possible explanation for this effect.
“The axion would be one billion times lighter than the electron,” says Dr Miguel Escudero Abenza, a theoretical physicist at CERN, near Geneva in Switzerland. “There could have been a lot of axions produced immediately after the Big Bang,” he says. So many, in fact, that despite the minute size of each axion, in total they could be responsible for dark matter and outnumber the mass of ordinary matter by more than five to one.
As axions are affected by gravity, they could have clumped together after the Big Bang into axion stars, just as ordinary matter would later coalesce into ordinary stars.
A single axion star would contain 10 billion trillion trillion trillion trillion trillion axions (1 followed by 70 zeroes). That’s more than the total number of atoms in the Sun. But because axions are so light, an axion star would only have a mass somewhere between those of Earth and Jupiter.
“An axion star also doesn’t look like a star,” says Abenza. “There’s no nuclear fusion – it’s just a stable bunch of axions.”
In an ordinary star, it’s the energy created by fusion pressing outwards that supports the star against gravitational collapse. The axions in an axion star, however, are in state known as a Bose-Einstein condensate, which gives them unusual properties. “If the axions try to collapse in further, they gain more speed,” Abenza says. This stops the collapse from happening and props up the axion star.
This mechanism means that axion stars would be considerably bigger than ordinary stars. In fact, they could grow to be twice as wide as the entire Solar System. Despite their huge size, axion stars would be very hard to spot because they wouldn’t emit any light. Nevertheless, Abenza has hit upon a way we might find them: by looking for what happens when they die.
Axion stars may be stable, but they don’t stay that way for long. “Within days an axion star can explode,” Abenza says. The process starts when a particle of light – a photon – hits an axion in the star.
This interaction causes the axion to decay into two more photons, which forces other axions to decay into yet more photons and so on in a relentless unstoppable cascade. “Very quickly this leads to the explosion of the whole system,” Abenza says. Such a detonation is known as a ‘bosenova’, as opposed to a supernova that marks the end of a massive star’s life.
As the axion is so light, the photons produced in a bosenova have incredibly low energy. This means they’re far beyond the visible part of the spectrum and instead would only be detectable by using radio telescopes. Luckily, there’s a giant radio telescope currently under construction that should be up to the task of spotting them: the Square Kilometre Array (SKA).
Due for completion in 2028, the SKA is being built across two far-flung sites, one in South Africa and the other in Australia. Once finished, it’ll have 197 radio dishes in South Africa’s Karoo region and 130,000 antennae in Murchison, Australia, which, taken together, will give the telescope the combined effective area of 1km 2 (approx. 0.3 miles 2 ).
Thousands of miles of electrical cables will enable the dishes and antennae at the two locations to act as if they were one giant telescope, making it 50 times more sensitive than any existing radio telescope.
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Dark matter's true identity
Using the SKA to find evidence of axion stars going bosenova will rely on something called the 21-centimetre line – one of the most important effects in radio astronomy. The 21cm line revolves around hydrogen, the most abundant element in the Universe.
A hydrogen atom contains a single electron orbiting a single proton. This is referred to as neutral hydrogen because the electrical charges of the electron and proton perfectly cancel each other out to zero. But the important thing is the electron in neutral hydrogen can ‘flip’, producing radiowaves that repeat every 21cm (approx 8in).
Lots of bosenovae could disrupt this process, however. “If only one billionth of all the dark matter exploded as bosenovae, the energy created would still match the surface temperature of the Sun,” Abenza says. That's about 5,500°C (9,930°F).
This extra heat would rip electrons away from hydrogen atoms, reducing the total amount of neutral hydrogen present. This effect could show up when the SKA makes measurements of the 21cm line in the early Universe.
“The signal will need to be strong to claim it’s dark stars, though,” says Abenza. Otherwise it could be put down to far more mundane phenomena.
Even if those measurements fail to find the signal of bosenovae and axion stars, it’ll still be useful in ruling out possibilities and refining the search for dark matter’s true identity. And that’s one of the main reasons to search for dark stars in the first place: to uncover what dark matter really is.
More often than not, dark matter is portrayed as a singular entity, whether it’s WIMPs or axions or something else. Yet that needn't be the case. “There’s no reason why it has to be one particle,” Font says. After all, you aren’t made of a single type of particle; you’re built from ordinary matter that’s made up of protons, neutrons and electrons. In fact, particle physicists know of 17 types of particles that together make up everything we see around us.
Perhaps dark matter is similarly diverse. Searching for the different versions of dark stars may well help us to say which particles are likely to exist and which aren’t. In turn that could reveal the intricate tapestry of the Universe to be a lot richer than we’ve been led to believe.
About our experts
Dr Andreea Font is a computational astrophysicist at Liverpool John Moores University. Her work has been published in Monthly Notices of the Royal Astronomical Society, Journal of Cosmology and Astroparticle Physics, and Springer Proceedings in Physics.
Dr Miguel Escudero Abenza is a theoretical physicist at CERN, near Geneva in Switzerland. His work has been published in Journal of Cosmology and Astroparticle Physics, The European Physical Journal C, and Proceedings of Science.
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