The extreme conditions found in neutron stars make them ideal laboratories for studying physics in conditions found nowhere else in the universe. Scientists also suspect that some dead stars made of the densest material in the known universe, called neutron stars, may act as traps for dark matter particles, which collide at high speeds and annihilate each other. New research suggests that brown dwarfs, also known as failed stars, may act as gravitational traps for dark matter, forcing this exotic material to interact with each other.
Scientists plan to use the ‘clocks’ of dead stars to shed light on the universe’s most mysterious entity: dark energy.
These timekeepers are actually pulsars, or rapidly spinning neutron stars born from the deaths of stars at least eight times the mass of the Sun. The extreme conditions found in neutron stars make them ideal laboratories for studying physics in conditions found nowhere else in the universe.
So-called “millisecond pulsars” can spin at hundreds of revolutions per second and emit beams of electromagnetic radiation from their poles, like cosmic lighthouses that pierce space. They got their name because when they were first spotted, these neutron stars appeared to pulse, increasing in brightness as their beams were aimed directly at Earth. The team’s findings were presented at the National Astronomy Meeting (NAM) 2024 at the University of Hull on July 15.
Diagram showing gravitational lensing caused by dark matter. Robert Lee
The ultra-precise synchronization of millisecond pulsar brightness changes allows them to be used as cosmic clocks in “pulsar timekeeping antennas.” These antennas are so precise that they can measure gravitational perturbations in the fabric of space and time, combined into a four-dimensional entity called “spacetime,” which could be an ideal way to search for dark matter.
“Science has developed very precise methods for measuring time,” said pulsar timekeeper researcher John Losecco of the University of Notre Dame. “On Earth, we have atomic clocks, and in space, we have pulsars.”
Dark matter is so mysterious because it doesn’t interact with light or normal matter, and if it does, it’s very weak and we can’t detect it. “Normal matter” is made up of atoms made up of electrons, protons, and neutrons, which interact with light and matter, so scientists know that dark matter must be made of other particles.
Despite its lack of interaction with light, dark matter exerts a gravitational force, and its presence can be inferred from the way that force affects light and, indeed, ordinary matter. It is the effect of this gravitational force on light that Losecco and his colleagues sought to study with pulsars.
Artist’s impression of an array of pulsars affected by gravitational ripples generated by a binary supermassive black hole in a distant galaxy. Aurora Simonnet/NANOGrav
According to Albert Einstein’s general theory of relativity, objects with mass curve the fabric of spacetime itself, and gravity arises from this curvature. When light passes through this curvature, its trajectory is also altered. This can change the time it takes for light to travel, causing light from the same distant object to reach Earth at different times, theoretically “slowing it down” (the speed of light doesn’t actually change, only the distance it travels).
Dark matter has mass, and therefore clumps of this mysterious form of matter can also bend space-time. Thus, the path of light from distant objects is bent and its arrival time is delayed when passing through clumps of dark matter. This effect is called “gravitational lensing,” with the intervening body that changes the path of the light being called a “gravitational lens.”
Losecco and his colleagues studied data collected from 65 pulsars in the Parkes Pulsar Timing Array. They found about 12 instances that indicated changes and delays in the pulsars’ timing, which is typically accurate to nanoseconds.
This indicates that the radio waves from these dead-star-like cosmic beacons are traveling through a warped space caused by an invisible concentration of mass somewhere between the pulsar and the telescope. Scientists speculate that these invisible masses may be “clumps” of dark matter.
“We take advantage of the fact that the Earth moves, the Sun moves, the pulsar moves, and even the dark matter moves,” Losecco said. “We see deviations in arrival times caused by changes in the distance between the observed mass and the line of sight to our ‘clock’ pulsar.”
This illustration shows a rapidly spinning neutron star surrounded by a blue magnetic field that emits jets of radio waves. NASA Goddard/Walt Feimer
The deviations observed by the team are quite small. To illustrate, a body the mass of the Sun would cause a delay of about 10 microseconds in the pulsar’s radio waves. The estimated deviations in the delay due to dark matter observed by the team are 10,000 times smaller.
“One of the discoveries indicates a distortion of the Sun’s mass by about 20%,” said Professor Losecco. “This object could be a candidate for dark matter.”
One side effect of the team’s research has been to improve the accuracy of data from the Parkes Pulsar Timing Array, which is being collected to search for evidence of low-frequency gravitational radiation.
Clumps of dark matter can add interference, or “noise,” to this data; identifying and removing this noise will help scientists better use the data set to search for low-frequency disturbances in spacetime called gravitational waves. This could reveal gravitational radiation from more distant, and therefore earlier, black hole mergers, and perhaps even background primordial gravitational waves left over from the Big Bang.
“The true nature of dark matter remains a mystery,” Losecco said. “This study sheds new light on the nature of dark matter and its distribution in the Milky Way, and may improve the accuracy of pulsar data.”
The scientists speculate that some dead stars made of the densest material in the known universe, called “neutron stars,” may act as traps for dark matter particles that collide at high speed and annihilate each other. In turn, the team says, the annihilation process likely heats up the dead stars from the inside.
Overall, dark matter is a tricky topic for researchers because, despite making up about 85% of the matter in the universe, it is effectively invisible because it does not interact with light. Dark matter also does not appear to interact with “normal matter” made up of protons, neutrons, and electrons, or if it does, these interactions are rare and weak. We have never observed them. This raises an interesting question: does dark matter interact with itself? A 2024 study by Bell and colleagues is available on the arXiv repository.
Nicole Bell, a theoretical physicist at the University of Melbourne, is interested in the self-interactions of dark matter, which occur in ordinary matter when an electron and its antiparticle, a positron, meet and annihilate, releasing energy. Dark matter is electrically neutral, meaning that any particles that make up the stuff could, in theory, have their own antiparticles.
NASA’s James Webb Space Telescope, shown in this artist’s illustration, can detect thermal radiation from neutron stars, indicating dark matter annihilation. NASA GSFC/CIL/Adriana Manrique Gutierrez
And, as with ordinary matter annihilation, when dark matter particles meet, dark matter annihilation should occur, and neutron stars could be the ideal extreme environment for such interactions.
Annihilation is when a particle and antiparticle collide and destroy each other. This is what would happen if dark matter were its own antiparticle, as is often assumed in the most widely studied models of dark matter. The capture and annihilation of dark matter in neutron stars would provide the heat that would keep the star from cooling too much.
This means that if neutron stars can act as “dark matter traps,” they could emit a thermal signature. If this can be detected, neutron stars could act as rudimentary “dark matter detectors,” helping scientists track this largely invisible form of matter.
A neutron star is born when a star at least eight times the mass of the Sun runs out of fuel to fuel nuclear fusion in its core. This ends the outward-pushing force of radiation pressure that has supported the star against the inward force of its own gravity for millions, sometimes billions, of years.
A cross-section of a neutron star filled with exotic quark matter. But could these dead stars hide something even more exotic? Jyrki Hokkanen/CSC
As a result, the star’s core collapses, causing a supernova explosion. This shock wave blows away the outer layers of the dying star, as well as most of its mass, leaving behind a core of a star with a mass of one to two times that of the Sun, which collapses to a width of about 20 kilometers.
Squeezing a body equivalent to more than half a million Earths into a body that would fit inside Chicago naturally has a tremendous effect on the material in the star’s core. Electrons and protons are smashed together, creating a sea of neutrons, particles that are normally found only in the core of atoms. This sea of neutrons, which makes up a neutron star, is so dense that if a tablespoon of it were dropped on Earth, it would weigh more than a billion tons. That’s about the same as Mount Everest.
Neutron stars are thus made of the densest matter in the known Universe, which is why scientists believe their gravitational pull may be strong enough to hold on to dark matter, which, despite not interacting with light and matter, does interact with gravity.
Bell explains that dark matter annihilation is thought to have happened frequently when the 13.8 billion-year-old universe was less than a second old, but is thought to be rare in the modern universe. The only exceptions are regions with large amounts of dark matter.
And if dark matter can indeed accumulate in the depths of neutron stars, Bell and his colleagues found that this could provide just the right dark matter-rich environment to allow it to annihilate in the old universe.
“Ultimately, there may be enough dark matter in a small area that significant dark matter annihilation can occur in these stars,” Bell said.
Illustration of a neutron star compared to Manhattan Island. NASA Goddard Space Flight Center
Bell added that in dark matter experiments in labs on Earth, scientists look for signals from dark matter particles interacting with normal matter, but neutron stars have a natural advantage in this regard.
In the lab, we look for dark matter particles colliding with atomic nuclei. But if that’s possible, then dark matter should also be able to collide with neutrons and protons in neutron stars. And neutron stars have a lot of neutrons.
In the study of neutron stars and dark matter, the enormous gravity of neutron stars may create another condition that makes self-interaction of dark matter particles inside these dead stars even more likely.
Dark matter is accelerated to speeds close to the speed of light when it falls into a neutron star. This is useful because it can increase the speed of interactions, potentially allowing us to study some types of dark matter interactions that are nearly impossible to observe in experiments on Earth.
Dark matter annihilation releases thermal energy into these dead star traps, so the team also looked at how long it would take for the neutron stars and their trapped dark matter to reach a state called “thermal equilibrium.” This is the point at which the two substances reach the same temperature and no more heat can be transferred between them.
This study showed that a neutron star filled with dark matter could reach thermal equilibrium in as little as 10,000 years, or as little as one year, depending on the model used. In cosmic terms, that’s just the blink of an eye.
To confirm this theory, researchers need to measure the temperature of neutron stars. Finding that these extremely dead stars are hotter than expected would confirm that dark matter particles are indeed annihilating in their cores. However, such a discovery would not be easy, since only older, cooler neutron stars emit thermal radiation that is not suppressed by other radiation. This would require the most powerful observational instrument humanity has ever launched into space: the James Webb Space Telescope.
(Left) An artist’s impression of the distribution of dark matter in a neutron star (right). NASA’s Goddard Space Flight Center/VOLKER SPRINGEL, MAX PLANCK INSTITUTE FOR ASTROPHYSICS
The neutron stars we are most interested in are very cold and difficult to see. “The temperature of these stars produces near-infrared radiation that could possibly be observed by the James Webb Space Telescope (JWST).
The lack of understanding of the nature of neutron stars may mean that this dark annihilation model would be easier to test on the type of stellar remnants that remain after the death of smaller stars like the Sun: white dwarfs.
Neutron stars would be good at capturing dark matter because of their extreme density. But they are also relatively poorly understood stars. “Similar ideas could apply to other stars that we understand better, like white dwarfs.
If the theory is correct, it will not only shed light on dark matter, but will also help scientists better understand the evolution of neutron stars.
Dark matter can be captured by stars and accumulate inside them. If this happens, it can also interact with itself and annihilate, releasing energy that heats the star.
“Dark dwarfs” may sound like the name of a new race of Sauron-worshipping Middle-earth inhabitants, but they are actually a new type of stellar body thought to exist at the centers of galaxies. The team’s research was published in the Journal of Cosmology and Astroparticle Physics (JCAP) in 2025.
The prefix “dark” here does not refer to the Dark Lord, but to dark matter, a mysterious substance that makes up 85% of the matter in the universe. This form of matter remains virtually invisible because it does not interact with light, but does interact with gravity.
New research suggests that brown dwarfs, also known as “failed stars,” may act as gravitational traps for dark matter, causing this exotic material to interact with one another. This releases energy, heating up these failed stars and turning them from brown dwarfs into dark stars. And the more dark matter these dark dwarfs accumulate, the more energy these stars emit on their dark side.
If this idea is correct and dark dwarfs do indeed inhabit the center of the Milky Way and other galaxies where dark matter is most common, it gives scientists a strong hint as to what hypothetical undiscovered particles might make up this mysterious form of matter.
Illustration of two brown dwarfs against the backdrop of the Perseus molecular cloud. Robert Lee
This is because only certain dark matter particles interact with each other and “self-destruct,” releasing energy. Perhaps the most striking example of such self-interacting particles are weakly interacting massive particles (WIMPs).
“Dark matter interacts gravitationally, so it can be captured by stars and accumulate inside them,” said team member Jeremy Sackstein of the University of Hawaii. “If that happens, it can also interact with itself and annihilate, releasing energy that heats the star.”
Brown dwarfs have earned the unfortunate nickname of “failed stars” because, although they form like stars from a cloud of collapsing gas and dust, they fail to accumulate enough mass to initiate the fusion reaction between hydrogen and helium in their cores.
This process powers main-sequence stars like the Sun. Brown dwarfs glow dimly because some nuclear fusion is occurring inside them, and because they are collapsing under their own gravity. But at the center of galaxies, these failed stars may have an alternative energy source.
“Dark dwarfs are objects with very low mass, about 8% of the mass of the Sun,” Sackstein explained. “These objects collect dark matter, which helps them become dark dwarfs.”
Dark matter interacts gravitationally but does not interact with other matter, allowing it to fall into the centers of galaxies without encountering much resistance. This means that the centers of galaxies are rich in dark matter and are therefore the most likely places to find dark dwarfs.
“The more dark matter there is around, the more of it you can capture,” Sackstein said. “And the more dark matter you have inside a star, the more energy will be produced when it annihilates.”
Dark matter can only power dark dwarfs by interacting with itself. Even if these interactions are rare or “weak” (which explains why we haven’t detected them yet), where dark matter is gravitationally compressed, as in the core of a brown dwarf, they could become frequent.
Illustration of dark matter engulfing a “dark dwarf” star. Robert Lee
The existence of dark dwarfs could therefore rule out non-interacting dark matter candidates, as well as particles that are too light. These may include the current leading dark matter candidate, axions.
“For dark dwarfs to exist, dark matter must be made of WIMPs, or any heavy particles that interact with each other so strongly that they produce visible matter,” Sackstein said.
Of course, all this would be pure speculation if the team couldn’t come up with a way to detect dark dwarfs and distinguish them from brown dwarfs, which don’t use dark matter, or ordinary stars.
Researchers propose a specific chemical marker that could be a telltale sign of dark dwarfs: the isotope lithium-7, which burns out easily and is therefore quickly absorbed by normal stars.
“There were several markers, but we proposed lithium-7 because it would be a really unique effect,” Sackstein said. “So if you could find an object that looked like a dark dwarf, you could look for this lithium, because it wouldn’t be there if it was a brown dwarf or something like that.”
The team believes that powerful telescopes such as NASA’s James Webb Space Telescope (JWST) are already capable of detecting cool, dim dark dwarfs.
Sackstein argues that if a dark dwarf is discovered at the center of the Milky Way, it would be “pretty compelling” evidence that WIMPs are dark matter particles.
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“With light dark matter candidates like the axion, I don’t think you’ll get something like a dark dwarf. They don’t accumulate inside stars,” the researcher said. “If we do find a dark dwarf, that would be strong evidence that dark matter is heavy and interacts strongly with itself, but only weakly with ‘normal’ matter. That includes the WIMP classes, but also some other, more exotic models.”
“Observing a dark dwarf does not give us a definitive conclusion that dark matter is a WIMP, but it would mean that it is either a WIMP or something that behaves like a WIMP in all respects.”
Does dark matter interact with itself? The answer may lie in vast clusters of colliding galaxies.
Dark matter is known to be asocial: it refuses to interact with light and “normal” matter, making it effectively invisible. But scientists aren’t sure whether dark matter interacts with itself.
If the particles that make up dark matter interact with each other, colliding and perhaps even destroying each other, a new study suggests that galaxy clusters could be used as natural dark matter colliders. This method of detecting dark matter would rely on two such vast groups of galaxies meeting and colliding.
Because these galaxy clusters are filled with dark matter, astronomers could study these sites of cosmic collisions to detect possible signs of dark matter interacting with itself. The team’s research was published in April 2024 in the journal Monthly Notices of the Royal Astronomical Society
“Galaxy clusters are also dominated by dark matter,” said Jacqueline McCleary, an assistant professor of physics at Northeastern University. “Eighty to 90 percent of their mass is dark matter, and the more massive the object, the faster the dark matter particles that make it up are moving. Essentially, we’re studying collisions at very high energies.”
This lack of interactions is one of the reasons dark matter is so difficult to understand. Despite making up more than 80% of the material universe and exerting a gravitational pull that literally keeps galaxies from flying apart, dark matter is effectively invisible because it does not interact with light. Likewise, dark matter particles slip through matter particles like cosmic ghosts.
The illustration shows a galaxy entangled in a cosmic web of invisible dark matter. Robert Lee
These antisocial properties led scientists to realize that dark matter could not be made of atoms made of electrons, protons, and neutrons that interact with light and with each other. This began the search for suitable dark matter candidate particles.
“Everything in the universe is made up of particles, waves, and fields, so the starting assumption is that dark matter must be a particle,” McCleary said. “The question is what kind of particle it is, because the concept of ‘particle’ is very vague.”
Candidates for these particles range from hypothetical “weakly interacting massive particles” (WIMPS) to “massive compact halo objects” (MACHOs) to tiny, nearly massless particles called axions or sterile neutrinos. Even tiny black holes left over from the Big Bang, called primordial black holes, are considered dark matter candidates.
The problem is that all these suspects are undetectable and thus remain frustratingly hypothetical.
The phantom nature of dark matter means that we can’t simply load dark matter particles into a particle accelerator on Earth, such as the Large Hadron Collider (LHC), to smash them together and determine their composition.
So while the LHC has unlocked some secrets of the so-called “standard model of particle physics” by splitting protons and observing the resulting streams of particles, the dark matter that lies beyond that model appears to be beyond the reach of the world’s most powerful particle accelerator.
The only way we can actually infer the presence of dark matter is by looking at the one interaction it appears to be involved in. Dark matter has mass, and so it curves spacetime (the four-dimensional unification of time and space), giving it a gravitational influence.
A view of the galaxy cluster Abell 1689, showing purple galaxies and bright stars that reveal vast amounts of dark matter. NASA
This influence can control the motion of ordinary matter and light, and we can detect it. It is through this indirect approach that we know that galaxies are surrounded by vast halos of dark matter, and that dark matter also exists in large quantities near the cores of galaxies.
Scientists also suggest that the grouping of galaxies into vast clusters is due to powerful strands of invisible dark matter that spread throughout the universe like an invisible web spun by a cosmic spider. This cosmic web also helps individual galaxies gain mass and grow, as its gravitational pull pulls on ordinary matter.
McCleary and his colleagues have proposed to abandon the artificial particle accelerator in favor of a natural one. They reason that if galaxy clusters and the dark matter they contain collide with sufficient force, detectable dark matter interactions could occur—if dark matter actually interacts with itself.
These interactions can be simple, such as dark matter particles colliding and bouncing off each other, or they can result in mutual annihilation and the release of energy. When particles and antiparticles, such as an electron and its antiparticle called a positron, meet, they annihilate each other, releasing the energy they contain. Some models of dark matter suggest that it is its own antiparticle, meaning that one form of interaction could be self-annihilation, which occurs when two dark matter particles meet.
To test this idea, the team created simulations of galaxy cluster collisions on computers powerful enough to probe the collisions down to the particle interactions. This required programming the simulations using physical models of how stars, gas, and dark matter interact.
Using this and other similar simulations, scientists will be able to build a roadmap for observing real dark matter particle collisions. From this, researchers will be able to gather clues that will help them determine some of the basic characteristics that a hypothetical dark matter particle should have. This could narrow down the range of possible dark matter particles.
“If you can measure some self-interaction or put some upper limit on it, you exclude or allow classes of particles that dark matter could be,” McCleary said. “If you measure some self-interaction, then you introduce a different class of particles. You allow a different class of theories.”
“This paper also suggests that if you study a set of, say, 100 merging galaxy clusters, you have a chance of measuring the self-interaction of dark matter up to some threshold.”
“It seems counterintuitive at first glance, because we always want to get a positive answer: What is it?” McCleary concluded. “But in this case, we’re kind of forced to ask: What isn’t it? It’s a process of elimination.”
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