IN THE early days – and we are talking very early, not long after the big bang – the universe might have been littered with strange stellar monsters. Wide enough to engulf our whole solar system, these stars would be powered not by nuclear fusion, like a regular star, but instead by dark matter: specifically, particles of this mysterious stuff self-annihilating to fuel so-called “dark stars”.
That is the idea, at least. But when , a theoretical astrophysicist at the University of Texas at Austin, first presented it at a conference in 2007, it didn’t go down particularly well. “I overheard some graduate students calling us crackpots,” she says.
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Regardless, the concept of dark stars has stuck with Freese. Over the past 16 years, she and her colleagues have refined their understanding of these tantalising hypothetical objects. The problem was, finding evidence for them always seemed out of reach.
Until recently, that is, because Freese and her colleagues have reported a potential sighting: unusual galaxies seen by a new telescope. “Maybe some of these objects aren’t really galaxies at all, but actually singular stars – dark stars,” says team member , then at Colgate University in New York.
Echoes of doubt still sound among other astronomers. “It’s a very controversial idea,” says , also at Colgate University, who led the team. But if they do exist, dark stars would not only be evidence for a specific kind of dark matter. They could also help crack one of the biggest problems in cosmology – the mysterious origins of the supermassive black holes that drive galactic evolution.
Our universe is awash with dark matter. We can’t see it directly, because it doesn’t interact with light, but we know it is there. It reveals itself by warping space-time, producing a lensing effect that distorts our view of other objects that we can see. “We have tons of evidence about the gravitational nature of dark matter,” says at the University of Utah. “It exists, and we know where it is.”
It is through these observations that we know dark matter ought to compose some 27 per cent of the universe. Around 68 per cent is the equally inexplicable dark energy that drives the universe’s expansion, with a mere 5 per cent being the normal matter that makes up chairs, tables and people.
Dark matter candidates
We don’t know precisely what dark matter is, although we have some ideas. In the late 20th century, a candidate called a weakly interacting massive particle, or WIMP, was introduced. Up to 1000 times more massive than protons, WIMPs wouldn’t interact with regular matter. But they would affect each other – and violently. Two such particles would annihilate on contact, producing a burst of energy in the form of gamma rays.
We can also infer how dark matter would have behaved in the early universe. After the big bang, around 13.8 billion years ago, the universe was filled with a morass of particles, with no complex structures. Dark matter moves more slowly than normal matter, so would have clumped together more easily under gravity. In the first 200 million years of cosmic history, huge conglomerations of dark matter called mini-halos formed. Eventually, these sucked in regular matter too, which formed stars and galaxies.
Freese’s realisation was that if these mini-halos were full of WIMPs, it could have led to an interesting process akin to the creation of a normal star. “We realised we had this process that the astronomy community had missed,” she says. It was an entirely new kind of star.
Stars form when a collapsing cloud of dust and gas made of regular matter condenses. Gravity squashes hydrogen and helium atoms together, eventually reaching the threshold at which nuclear fusion begins and the core of a star is forged. In stars, the force of gravity pushing inwards is exactly balanced by the force outwards from fusion.
But if the density of dark matter were high enough in some halos, annihilating WIMPs might produce enough energy that the outward force would prevent normal matter reaching the critical density. Nuclear fusion would never start – instead, you would have a different class of object that would stabilise at a far greater size, perhaps with a diameter similar to that of Saturn’s orbit and a mass up to a million times that of the sun. “There’s this heat source which prevents that cloud from collapsing any more,” says Freese. “They’re powered by dark matter.”
The dark matter content of the stars would be low, just 0.1 per cent. But that would be enough to produce trillions upon trillions of annihilations per second, making the star shine incredibly brightly in brilliant white or blue. In that sense, “dark star” is a misnomer. In the most extreme cases, dark stars could grow truly gigantic and outshine even entire galaxies in the early universe. “They can get a billion times as bright as the sun,” says Freese.
Freese published a series of papers on dark stars, the , written with her colleagues Douglas Spolyar and Paolo Gondolo, appeared in 2008. at the University of Chicago remembers there was “intense debate” when they were published. “I would go to talks and hear people yell at each other,” he says. “I didn’t know who was right and wrong.”
The idea has some support. “I think it’s a very cool idea that’s absolutely possible,” says Sandick. It is also far from the only exotic star that has been proposed (see “The strangest stars”, below).
There is good reason to hope that these theoretical stars do exist. Aside from being an entirely new kind of star, dark stars could offer a solution to a big cosmological mystery: the origin of supermassive black holes. These are the monster black holes, so dense that even light can’t escape them, that sit at the centre of galaxies.
The supermassive black hole puzzle
When we look at the distant, early universe, we see galaxies younger than a billion years old with supermassive black holes that are a billion times the mass of our sun at their centres. How these black holes grew so quickly is a puzzle. “The emergence of such massive black holes in the very early universe is a big problem,” says at the United Arab Emirates University. They may have grown from merging smaller black holes, but there doesn’t appear to have been enough time for that to happen.
Dark stars could be the answer, in that they could be the elusive seeds of supermassive black holes. They would mostly have lived fast and died young, lasting only millions of years. Smaller dark stars, perhaps 100 times or so the mass of our sun, might be reborn as regular stars once WIMP annihilation ceased. “Once their dark matter fuel runs out, then fusion might kick in,” says Freese.
But all dark stars, once they ran out of fuel entirely, would collapse into black holes. In the case of supermassive dark stars, given their huge masses, the resultant black holes would be equally massive, up to a million solar masses. “We actually solve the outstanding big black hole problem,” says Freese. “If you start with our dark stars, you’ll have million-solar-mass seeds that could merge together to make billion-solar-mass black holes.”
During the 2010s, however, the case for dark stars hit a snag. Other candidates for dark matter emerged and the case for WIMPs weakened. With no evidence for WIMPs, lighter theoretical particles called axions became many physicists’ bet for explaining dark matter. And axions would be too light to produce enough energy to form dark stars. Others, like at Queen Mary University of London, favour the idea that dark matter isn’t made of mysterious particles at all, but consists of black holes created in the early universe, called primordial black holes.
Yet Freese remained bullish about the prospects of dark stars, and now things are looking up. In 2022, she and her colleagues calculated that . “It doesn’t have to be WIMPs,” she says. There is an alternative where dark matter is still envisaged as a particle, but one that interacts more strongly with itself. This self-interacting dark matter concept is popular at the moment, says Freese.
Then, earlier this year, the furthest-seeing eye in the sky spotted something.
Hints of dark stars
The James Webb Space Telescope (JWST), launched in December 2021, is able to peer back deep into the universe’s history thanks to its large gold-plated mirror that observes in infrared light. It has already seen galaxies dating to the first few hundred million years of the universe, earlier than any galaxies seen by any other telescope. Those results have shown more bright objects in the early universe than expected. “There’s no doubt that the results of JWST do indicate that something strange is going on,” says Carr. Galaxies weren’t expected to grow much until a billion years or so after the big bang, but JWST seems to have found lots of galaxies forming much earlier.
In June, astronomers on the JWST Advanced Deep Extragalactic Survey (JADES) announced a handful of new candidate galaxies that were the earliest yet, with one dating to just 320 million years after the big bang. Freese and her colleagues, however, put out – a study yet to be peer-reviewed – suggesting that these might not be galaxies. Instead, they think the objects, appearing as red smudges to JWST, are individual dark stars.
Their reasoning is that three of the objects appear rounded, like a star, and lack the wispy features of a galaxy. The objects also fit the model of dark stars, with the amount of light produced consistent with what would be expected. Two of the candidates are estimated at a million times our sun’s mass, and one of them at half that.
It is still possible that these will turn out to be very small galaxies, perhaps appearing like individual point sources because of a lack of resolution. “We still prefer the simpler solution,” says at the University of Cambridge, who is part of the JADES team. “At the moment, there is no clear evidence for a dark star model.” at the University of Arizona, also part of the JADES team, is similarly cautious, noting that two of the three candidates did appear to have wispy features consistent with a small galaxy, while the third could simply be a compact galaxy. “To claim something is a dark star, you really need to have pretty solid proof,” she says.
There is a chance we might get it. In October, JADES will gather the second half of its data, including detailed spectra of these objects, which means measuring the light they emit across a range of wavelengths. We should be able to pick up the signature of an ionised form of helium, known as helium-II. If the mystery objects are dark stars, astronomers would expect to see the helium-II absorbing light at a specific wavelength. If, on the other hand, they are galaxies, they would expect helium-II to emit light at that wavelength. “The smoking gun signature of a dark star is the helium-II feature,” says Paulin.
There may be more dark star candidates, too. Ilie says he is working on a new paper with fresh sightings from JWST observations. “We’re not going to be lacking candidates,” he says. Only JWST has the power to find dark stars at the moment, although the likes of NASA’s upcoming Roman Space Telescope, set to launch in 2027, might also have what it takes.
For the majority of astronomers, the idea of dark stars remains hard to swallow. “Most people prefer to be conservative and not invoke some weird thing like dark stars,” says Carr. But then again, our cosmos is littered with unlikely objects, from black holes to magnetars. “We’re surrounded by weirdness,” says Carr. Why not dark stars, too?
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THE STRANGEST STARS
Dark stars (see main story) are far from the only exotic stellar phenomena we may or may not ultimately discover in space.
JANUS STARS
In July, astronomers said they had spotted a star that was apparently split down the middle, with one half made of helium and the other made of hydrogen. It had two faces, like the god Janus in Roman mythology. It now seems this star may have just been caught at a strange moment, right in the middle of transitioning from burning helium to hydrogen, a process many white dwarf stars are thought to go through.
HYBRID STARS
Also known as Thorne-Zytkow objects, these consist of a neutron star within a regular one. They are the Russian dolls of stars, if you will. They probably form when two stars collide and merge. During their short lives, they would look indistinguishable from other large stars. These remain hypothetical – the only way to discover one would be through a combination of gravitational wave measurements and observations of the elements within the star.
BOSON STARS
When is a black hole not a black hole? When it is a boson star. Stars are normally made of fermions, the particles that make up matter. But it is possible for bosons, the force-carrying particles of nature, to cram together in almost unlimited numbers and create a transparent object that does little except exert a huge gravitational pull on everything around it. These objects would be almost indistinguishable from black holes – which is why astronomers haven't yet devised any easy ways of finding them.
Jonathan O’Callaghan is a freelance writer based in the UK
