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For​ more than fifty years, physicists have been stumped by dark matter. Careful measurement of a range of phenomena, from the motion of enormous clusters of galaxies to the rate at which individual galaxies spin, have indicated that all the stuff astronomers can see – the trillions of stars dotted across the night sky – contributes just a fraction of the total mass of the universe. The observations suggest that ‘missing mass’ exerts a gravitational pull on visible matter, altering the paths of the objects that we can see. The mysterious matter doesn’t light up on its own; it remains dark. And there is a lot of it: for every kilogram of matter visible throughout the cosmos, more than five kilograms of dark matter seem to lurk unseen.

Dark matter – whatever it is – played an essential role in the development of the universe. It was thanks to dark matter that pockets of ordinary matter began to clump into stars and galaxies soon after the Big Bang; without that added gravitational effect, the rapid expansion of the universe would have diluted ordinary matter before such structures could have formed. No dark matter, no stable galaxies; and without stable galaxies, like our own Milky Way, no humans to search the sky and wonder.

Physicists’ first solution, when originally confronted with the puzzle of dark matter, remains the most popular: perhaps some new, hypothetical elementary particles exist – cousins to the familiar electrons and quarks – which interact via gravitation but remain impervious to light. Over the years, various contenders have been proposed, from WIMPs (weakly interacting massive particles), which might weigh anything between ten thousand and a million times more than an electron, to the pipsqueak ‘axions’, which might be trillions of times lighter than an electron. In fact, both WIMPs and axions were first posited to address other riddles in particle physics, but before long, physicists recognised that if such particles really were skittering around the cosmos, they would behave just like dark matter.

There’s just one snag: after decades of meticulous experiments, no clear evidence has turned up that any such particle exists. Buried deep beneath the Gran Sasso mountains in northern Italy, for example, the XENON collaboration has been surveilling huge vats of liquid xenon for about twenty years, looking out for the telltale flashes of light that should occur when a xenon nucleus is struck by an incoming WIMP. The researchers have pushed the sensitivity of their detector to record levels, yet no WIMPs have been found. Meanwhile, other projects, such as the Axion Dark Matter Experiment (ADMX) based at the University of Washington, have been trying to catch an axion. When it traverses a strong magnetic field, this hypothetical particle should convert into pairs of photons. Yet after thirty years of dedicated searching, aided by remarkable improvements in magnet and detector technologies, not an axion in sight.

Others have wondered whether taking account of dark matter requires that the laws of gravity themselves be altered. For more than a hundred years, physicists and astronomers have tried to make sense of the universe by using Einstein’s general theory of relativity. It explains all the phenomena we associate with gravity – from the fall of an apple on Earth to the swirling motions of distant galaxies and beyond – in terms of the warping of space and time. Large clumps of matter distend spacetime much as a bowling ball warps a trampoline; this curvature, in turn, bends the paths of nearby objects away from the straight and narrow. Predictions based on Einstein’s relativity have withstood every test that physicists and astronomers have been able to concoct. And when attempts have been made to modify general relativity to accommodate, say, data on various galaxies’ rates of spin, these changes have typically introduced inconsistencies between theory and observations of other phenomena.

A third possibility remains, equal parts audacious and mundane. What if dark matter is just ordinary matter locked inside black holes – from which, after all, light cannot escape. Such massive, dark objects would trundle around the cosmos, nudging the motion of visible matter while themselves evading direct detection. No need to speculate about hypothetical particles with exotic properties; no need to wreck the rules of relativity. The idea, in outline, is not new, but it has attracted increasing attention in the scientific community over the past decade.

Einstein himself resisted the notion of black holes, though eventually physicists came to see them as a robust prediction of relativity. Among the most important clarifications came from J. Robert Oppenheimer, who was teaching theoretical physics at Berkeley in the 1930s when he and a graduate student, Hartland Snyder, worked out what would happen to a star after it exhausted its nuclear fuel. With no more outward-directed pressure coming from nuclear reactions in its core, they concluded, a massive star would collapse in on itself. The dense concentration of matter that remained would severely deform the surrounding spacetime, trapping even light-rays. Their paper, treated at the time as a theoretical curiosity, appeared in the Physical Review on 1 September 1939, just as Nazi tanks rolled into Poland. Before long, Oppenheimer was swept up in the nascent nuclear weapons project; neither he nor his student published on black holes again.

Years later, in a paper published in 1966, the Soviet astrophysicist Yakov Zeldovich wondered whether black holes might have formed soon after the Big Bang. Zeldovich, a long-time leader of the Soviet nuclear weapons programme, had a clearer understanding than most of the way matter behaved under extreme heat and pressure. Thanks to an intensive journal-translation effort launched in the 1950s – with secret underwriting by the US Air Force and CIA – Zeldovich’s article was republished in English in 1967, though it didn’t find many readers in the West.

Stephen Hawking independently broached the idea in a brief, crisp paper from 1971 demonstrating, more explicitly than Zeldovich, that black holes could have formed very early in the history of the universe. He first noted that ‘ordinary’ black holes, of the sort that Oppenheimer had considered, would result from the collapse of a star, and that their mass would have to be roughly equal to the mass of the Sun. In contrast, primordial black holes – a distinct type that could have formed immediately after the Big Bang – would bypass stellar evolution altogether, forming directly from the gravitational collapse of some local lumpiness in the early distribution of matter. As Hawking emphasised, such a direct collapse meant that primordial black holes could form with an enormous range of masses, either much smaller or much larger than the mass of the Sun. Hawking even suggested that primordial black holes – having formed long before the first stars or galaxies – might play the role of dark matter. He pursued the idea at Cambridge in the 1970s, but few paid much attention. Black holes of any type still struck most physicists and astronomers at the time as a speculative curiosity.

By the mid-2010s, they were no longer regarded this way. Astronomers had collected indirect evidence since the 1970s that enormous black holes might lurk at the centre of most galaxies. They suspected that such ‘supermassive’ black holes might have formed from the collapse of ordinary stars long ago – exactly as Oppenheimer had described – and then grown bloated over time by gobbling up huge amounts of matter from their surroundings. But a few years ago the picture changed dramatically. In February 2016, the international LIGO-Virgo-KAGRA Collaboration, consisting of more than a thousand researchers across 133 institutions on five continents, announced the first successful detection of gravitational waves. Einstein himself had predicted that objects’ violent motions should excite tiny ripples in the taut fabric of spacetime. Yet evidence of these ripples remained elusive for the next hundred years, until the LIGO-Virgo team first measured them using a pair of L-shaped detectors with legs four kilometres long. Early in the morning on 14 September 2015, the detectors in Louisiana and in Washington state rang in perfect synchrony – once you took into account the time it takes for a gravitational wave, washing over the Earth at the speed of light, to cross the distance between them. The wave’s specific pattern indicated that it had originated in the cataclysmic collision and merger of two large black holes far beyond the limits of our own galaxy.

Three years later, another globe-spanning collaboration, the Event Horizon Telescope team, released the first composite image of the immediate vicinity of a black hole, a gargantuan entity at the centre of galaxy M87, more than fifty million light years from Earth. The swirl of visible matter and radiation revealed the shadow of a black hole about 6.5 billion times more massive than the Sun.

These dramatic observations renewed physicists’ interest in primordial black holes. As Hawking had emphasised in the 1970s, stellar-collapse black holes form with masses comparable to that of the Sun, whereas primordial black holes could form with a large range of masses. Both the black holes that had caused the LIGO-Virgo gravitational waves and the monster black hole in M87 were much more massive than the Sun; especially in the latter case, astrophysicists have struggled to put forward any plausible mechanism by which a solar-mass black hole could have grown so large over the timescale involved. In the past few months, data from the James Webb Space Telescope has been used to identify other black holes that seem to be much too large and much too old to be consistent with known stellar-formation processes.

These whoppers boost the prospect that primordial black holes might really exist. But they are awkward candidates for dark matter. If dark matter consisted of such objects, they should be straying across astronomers’ lines of sight with some regularity. Since black holes warp the spacetime around them, their transit across the axis between an observer and a distant star results in a temporary distortion in the star’s brightness – an effect known as ‘gravitational lensing’. But in 2019, astronomers using the Subaru Telescope on Mauna Kea, Hawaii showed, by observing tens of millions of stars in the nearby Andromeda galaxy and recording the frequency of lensing events, that primordial black holes of a mass comparable to or greater than that of the Moon could account for no more than 1 per cent of all dark matter. Indeed, the implication was that if primordial black holes were to account for dark matter, their typical mass could be no greater than about one ten-billionth that of the Sun.

The lensing survey left open the possibility that dark matter might consist of smaller primordial black holes. However, they can’t be too small. The reason relates to the work Hawking did soon after writing his first papers about primordial black holes, work in which he made his landmark claim that, because of certain quantum-mechanical effects, it was possible that black holes emitted radiation after all.

Since the 1930s, quantum physicists had predicted, on the basis of Werner Heisenberg’s uncertainty principle, that pairs of tiny particles – particle and antiparticle – must flit in and out of existence all the time. Having temporarily ‘borrowed’ excess energy from empty space, such pairs of particles must square their accounts, recombining with each other and winking back out of existence on a short timescale set by Heisenberg’s relation. Such quantum fluctuations, first observed at Columbia University in the late 1940s, are now routinely measured in precision experiments; the phenomena match predictions from quantum theory all the way out to twelve decimal places.

Hawking reasoned that if such a particle pair were to form near a black hole, one of them might fall into the black hole before the pair could recombine. Since nothing – not even a wayward quantum particle – can escape from a black hole, the particle left outside would not then be able to recombine with its partner to ‘pay back’ the energy that together they had borrowed. Hawking suggested that instead the gravitational field of the black hole would supply the balance of energy, leaving the abandoned particle free to jet off. To a distant observer, this process would appear as if the black hole itself had radiated the particle. As Hawking summarised in his book A Brief History of Time, ‘Black holes ain’t so black.’

Hawking​ emission would be much too weak to measure for large-mass black holes, but it should have a more pronounced effect in the case of small-mass black holes. What’s more, a tiny black hole should shrink as it lends energy to the abandoned particles; and the smaller its mass, the more efficiently it should radiate, shrinking at an ever faster rate until it has evaporated away completely. The late-stage emission process should spew out energetic charged particles as well as high-energy gamma radiation.

Incredibly, some of the most important evidence concerning any possible relationship between dark matter and small-mass primordial black holes comes from equipment that is nearly as old as Hawking’s prediction. Back in September 1977, Nasa launched the Voyager 1 spacecraft to probe the outer solar system and beyond. After it had beamed back spectacular images of Jupiter and Saturn, the tiny craft continued on its path further and further away from the Sun. In August 2012, Voyager 1 officially left the solar system. Since then, it has been beyond the influence of the Sun’s solar wind and magnetic field, and its onboard particle detectors – humble little 1970s devices – have been immersed in a flux of interstellar cosmic rays.

If our universe were filled with ultra-low-mass primordial black holes – enough of them to account for all the dark matter in the cosmos – there should be a steady thrum of high-energy charged particles criss-crossing empty space, the late-stage emission products from black holes undergoing Hawking emission. In that case, Voyager 1 should by now be awash in such particles. Right up until the closing weeks of 2023 (when a glitch in its onboard computer temporarily interrupted its communications), it dutifully continued its reports, each signal taking eighteen hours to reach Earth. The counts of charged particles it detected remained low enough to rule out the presence of a large population of ultra-low-mass primordial black holes. And, basing their calculations on the connection between the Hawking emission rate and the mass of a black hole, physicists have used the Voyager data to place a lower bound on the mass of primordial black holes that could comprise dark matter: no smaller than ten million billion times less than the mass of the Sun.

These two groups of observations – modern lensing surveys and particle counts logged on the rickety Voyager space probe – thus delimit a range of masses within which microscopic primordial black holes could account for all of dark matter: no larger than one ten-billionth the mass of the Sun, and no smaller than one ten-million-billionth the mass of the Sun. To press further, several research groups (including my own) have proposed turning our local cosmic neighbourhood into a vast high-precision dark-matter detector. Astronauts on three Apollo missions – beginning with the Apollo 11 landing in July 1969 – placed special reflectors on the surface of the Moon. Unpiloted craft from the Soviet Union in the early 1970s and, in July last year, from India added several more. Within days of the first installation, astronomers on Earth began directing lasers from ground-based observatories to the lunar reflectors and carefully timing the arrival of the return signals. Since 2007, these efforts have achieved millimetre-level precision in measuring the distance between the Earth and the Moon. Meanwhile, telemetry with Mars orbiters and rovers over the past twenty years has enabled astronomers to routinely measure the Earth-Mars distance to within ten centimetres. Closer to home, the dozens of satellites in medium-Earth orbit that comprise the Global Positioning System (GPS) network have been tracked to within a centimetre, moment by moment for decades.

Given all this data, we may ask: are there any hints that a tiny primordial black hole, with a mass within the prescribed range for dark matter, has flown through the inner solar system? A flyby from a microscopic primordial black hole would set visible objects wobbling, just a tiny bit at first, but more and more over time and with a particular pattern. The effect would be subtle, but could be just large enough to register amid the glut of high-precision tracking data for nearby satellites, the Moon or Mars.

If dark matter really does consist of tiny primordial black holes, then such a flyby should have occurred about once every ten years. Knowing how a visible object’s wobbles from such an event change over time, we can now sift through decades of data to search for specific types of anomaly – hints that Mars, say, was just a little bit off from the location it should have been, if no black holes had sped by and altered its path. We can pose the same question prospectively, collaborating with astronomers who already track the motions of objects in the solar system, keeping a lookout for unexpected shifts in various objects’ locations over time. Compared with the decades-long drought from experiments designed to detect hypothetical dark-matter particles, the task of searching astronomical data for tiny wiggles in the motion of Mars seems downright concrete. By combining ageing Cold War infrastructure – space probes, lasers, GPS – with otherworldly notions of warped spacetime, researchers around the world may soon discover the nature of the mysterious matter that shapes our universe.

David Kaiser is a professor of physics and the history of science at MIT. His most recent book, Quantum Legacies, appeared in 2020.

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