According to the research team, their results also allow them to more accurately determine what proportion of the energy and matter in the Universe is dark energy. They reached their conclusions after analyzing observations of galaxy clusters made by the eROSITA X-ray telescope, which scans the entire sky above Earth in search of detectable galaxy clusters. eROSITA is mounted on Spektr-RG, a Russian-German space telescope launched into Earth orbit in 2019.
Physicists first proposed the existence of dark matter in 1933, in part because equations showed that galaxies lacked observable matter to prevent them from disintegrating. Furthermore, the observed rotation rates of galaxies did not match the expected results of standard physics models.
Further research was largely delayed until the 1970s, when more sophisticated scientific instruments, from gamma-ray detectors to space telescopes, allowed astronomers and physicists to confirm earlier calculations and observations.
Powerful radio telescopes have also provided clues, such as gravitational lensing (an effect in which matter causes light to bend between the source and the observer), and have provided compelling evidence for the existence of some matter we can detect but not see. The key is that we can’t see it; it doesn’t interact with light.
Everything you see, everything you feel, everything you are made of makes up only 5 percent of the universe, the rest is dark matter.
Physicists have calculated that about 27 percent of the entire universe consists of dark matter, and the rest (68 percent) is an equally mysterious phenomenon called dark energy.
The invisible part of the Universe consists of two components: dark energy and dark matter. The latter is somewhat less mysterious, and scientists are virtually certain of its existence: without it, the laws of gravity simply wouldn’t work. Galaxies as we see them would not have been able to gather to their current sizes in the time since the Big Bang if only visible matter existed. Moreover, many galaxies wouldn’t even be able to hold together if the gravity of visible matter were the only force attracting them together.
The new European space telescope, Euclid, will attempt to map this invisible matter, but what if the results don’t meet expectations? Much is at stake, including Einstein’s famous and widely accepted theory of general relativity.
However, the situation with dark energy is more complex. The discovery of this elusive force dates back to 1998, when astronomers discovered that the expansion of the universe was accelerating. This expansion was initially driven by the energy of the Big Bang, but because the Big Bang occurred so long ago (13.8 billion years ago, to be precise), this expansion should be slowing.
To solve this mystery, theorists have proposed the existence of a mysterious force—dark energy—that resists gravity and pushes matter apart. According to cosmological models, dark energy makes up 68% of all energy in the cosmos, according to NASA. However, astronomers acknowledge that the evidence for its existence is rather vague.
Astronomers hope to detect traces of dark energy in the distribution of galaxies in space mapped by the Euclid mission.
Models show that this energy should be uniformly distributed throughout the universe and that its intensity has always been the same. At the dawn of the universe, the influence of dark energy was not so noticeable, as the initial “kick” from the Big Bang drove most of the universe’s expansion. However, about 5-6 billion years ago, dark energy became the dominant force, counteracting the gravity of both ordinary and dark matter.
However, there’s also the possibility that dark energy doesn’t exist at all, and that the cosmological models that predict its existence are incorrect. These models are based on Albert Einstein’s general theory of relativity, which describes the physical “rules” of the universe using a series of equations. If future observations by the Euclid telescope (and other telescopes designed to study dark energy) reveal that dark energy isn’t such a constant and omnipresent force, it would mean that this famous theory isn’t entirely correct.
Euclid will search for evidence of dark energy by mapping the distribution of galaxies back 10 billion years and comparing how this distribution has changed over the evolution of the Universe. These observations will reveal whether there truly is a constant, omnipresent force, or whether something else is at work.
Broadly speaking, we can measure matter and energy in the Universe by observing them in one of four interactions:
— With electromagnetic radiation (light)
— Through gravitational effects
— With other matter through the strong nuclear force, which holds matter together
— The weak nuclear force, or the interaction of subatomic particles, responsible for radioactive decay
Dark matter eludes most of these observations because it doesn’t appear to interact with ordinary matter at all, except through gravity. But that doesn’t stop physicists from ruling out other methods.
This corridor leads to the Jadugoda Underground Research Laboratory in India. Located 560 meters below the surface, this laboratory will search for weakly interacting massive particles (WIMPS), which indicate the existence of dark matter.
One of Brown’s areas of research is studying the interactions of dark matter with ordinary matter in the form of liquid xenon isotopes. Xenon-124 has a half-life roughly a trillion times the age of the universe. Enormous reservoirs of this substance are pumped deep into boreholes in the Earth’s crust to limit background noise, such as electromagnetic radiation, which could interfere with measurements. Only dark matter and certain subatomic particles, such as muons and neutrinos, can pass through thousands of feet of dense rock.
Theoretically, only the exceptionally slow natural radioactive decay of xenon-124 or interactions with muons, neutrinos, or dark matter could cause any changes in the isotope. If a subatomic dark matter particle knocks an electron out of xenon-124, the Xenon1T experiment is expected to detect it.
The experiment is being conducted deep underground to avoid exposure to electromagnetic radiation. XENON1T
While dark matter scientists have yet to detect direct interactions with the elusive subatomic particles, they have certainly made a number of other interesting observations, including the decay of xenon-124 – the rarest event ever recorded in human history.
We know more about what dark matter isn’t than what it is. First, it’s not dark energy. It’s a form of energy, the existence of which has also been indirectly proven, but which likely exists because the universe is expanding at an accelerating rate, contrary to the laws of physics governing ordinary matter and energy.
And dark matter is not antimatter, which is ordinary matter composed of subatomic particles with a charge directly opposite to that of matter. When antimatter and matter collide, they annihilate, generating bursts of gamma radiation. Dark matter can also emit gamma radiation when it collides with its counterpart, dark antimatter, forming ordinary matter.
And finally, dark matter is not just another class of the three families of ordinary matter, such as hadrons, leptons, or bosons, the last two of which were previously theoretical but have finally been directly detected in particle accelerators and behave differently than we expect.
However, leptons and bosons provide us with a clue as to where we can go from here. Dark matter appears to be a form of matter composed of an entirely different class or classes of subatomic particles. One of the most promising is the WIMP (weakly interacting massive particle).
Despite their vague name, WIMPs are thought to have a mass 1,000 times greater than the mass of protons in ordinary matter. And the theory describing how WIMPs work is perfectly consistent with calculations of the amount of dark matter in the Universe.
But WIMPs are far from the only theory that can be considered. There are also primordial black holes, which are essentially small black holes left over from the Big Bang. However, researchers have not observed gravitational microlensing in them, ruling out the possibility that some of the mass in primordial black holes could be dark matter.
Then there are theoretical considerations about particles like SIMPs and axions, and countless other potential clues.
Of course, conducting research when you can’t observe what you think exists, or doesn’t necessarily exist, can be quite tedious. For example, researchers from Yale University have discovered two galaxies that contain no dark matter at all. This research points to some intriguing possibilities for how dark matter functions in the Universe: dark matter interacts with ordinary matter through a mechanism we currently don’t understand—the so-called “dark force,” or fifth force in the Universe.
A composite image from optical and X-ray telescopes, with probable dark matter highlighted in blue, although it is not directly visible in the images. Peter van Dokkum/Yale University
Another idea is that dark matter interacts through more known forces than just gravity, but does so with such a weak interaction strength that we simply don’t yet have the means to reliably detect signals.
Over the past 50 years, particle physics has focused on breaking the universe down into its smallest components. Currently, dark matter doesn’t fit into some ideas about how the universe works, particularly the standard model of particle physics.
Furthermore, fundamental particle physics, including the search for dark matter, has already yielded significant technological advances. Many of the detection tools used in this field are widely applicable to other fields, such as medical imaging or nuclear safety.
The internet, for example, was created in part because theoretical physicists at CERN wanted to find new ways to exchange data. Meanwhile, GPS relies to some extent on Einstein’s general theory of relativity, which explains how gravity bends space and time.
Dark matter appears to be distributed throughout the cosmos like a network, with galaxy clusters forming at the nodes where the filaments intersect. Dark matter doesn’t interact with light, so we can’t see it directly. Instead, scientists detect its presence through its gravitational influence on visible objects.
Here are some key pieces of evidence for the existence of dark matter:
1. Galaxy rotation curves
When astronomers measure the rotation speed of stars around the centers of galaxies, they discover something strange. Stars at the edges of galaxies move much faster than they should if only visible matter were present. This suggests that something invisible—dark matter—is increasing the galaxy’s mass and preventing these stars from escaping into space.
2. Gravitational lensing
According to Einstein’s general theory of relativity, massive objects can bend the path of light. As light from distant galaxies travels toward us, it sometimes passes near invisible clumps of dark matter, which bend the light’s path through a phenomenon called gravitational lensing. By studying how light is distorted by galaxy clusters, astronomers have been able to map the dark matter in the universe.
3. Relic cosmic radiation (RCR)
The cosmic microwave background (CMB) is the remnant radiation from the Big Bang that contains tiny temperature fluctuations that reveal the distribution of matter in the early universe. The nature of these fluctuations only makes sense if dark matter already existed and influenced how ordinary matter condensed.
4. Large-scale structure of the Universe
Computer simulations of the evolution of the Universe show that galaxies and galaxy clusters form in this way only in the presence of massive amounts of dark matter. Without it, structures would be much smaller and more scattered.
A mysterious glow emanating from the center of the Milky Way may be caused by the destruction of dark matter. Mattia Di Mauro (ESO/Fermi-Lat)
Here are some of the leading candidates for dark matter:
1. WIMP (weakly interacting massive particles)
These hypothetical particles are among the leading contenders. They possess mass and interact via gravity and possibly the weak nuclear force, but not light or electromagnetism, making them invisible. Large underground detectors are attempting to detect one of these elusive particles in action, but so far, they have been unsuccessful.
2. Axions
Axions are ultra-light particles that can exist in large numbers and act as a cold, invisible fluid in space. They are another strong candidate for dark matter, particularly because they may help solve other physics mysteries, such as the strong CP composition problem in particle physics.
3. Sterile neutrinos
Neutrinos are ghostly particles that barely interact with matter. Sterile neutrinos would be even more elusive than ordinary ones, and if they exist, they could partially or completely explain dark matter.
4. Primordial black holes
Some scientists speculate that dark matter may consist of tiny black holes that formed in the early universe. These would be small and difficult to detect, but their existence remains unlikely.
A new experimental technique has put constraints on a potential dark matter candidate called “dark photons.” The new method could help scientists shed light on the universe’s most mysterious substance by narrowing the search for a specific dark matter candidate—the hidden “dark photons.”
Dark matter makes up about 85% of the matter in the universe, but because it doesn’t interact with light, or interacts very weakly, it remains virtually invisible. The fact that dark matter doesn’t appear to interact electromagnetically means scientists know it can’t be composed of the atoms that make up “ordinary” matter—the stuff that makes up stars, planets, and our bodies. The team’s research was published in June 2023 in the journal Physical Review Letters.
The mystery of dark matter is such a pressing issue for scientists because it means that observable matter accounts for only 15% of all matter in the cosmos, not counting energy. This has led to a search for potential dark matter candidates, such as so-called “hidden” or “dark” photons.
These dark photons will differ from ordinary photons, the massless particles that make up light, because dark photons are theoretically expected to have mass. However, the mass of dark photons will be negligible, approximately twenty orders of magnitude smaller than the mass of an electron. It is this ultra-light nature that makes dark photons good candidates for dark matter, but also makes them incredibly difficult to detect.
Dark photons were initially proposed as candidates for dark matter because they theoretically interacted weakly with ordinary photons, meaning they could have played a role in heating the early universe. This explains why the cosmic web, the large-scale structure in the universe that connects galaxies, was hotter than predicted by observations by the Hubble Space Telescope.
Researchers from the California Institute of Technology (Caltech) have developed a new method for detecting dark photons. While this new strategy hasn’t yet detected a single hypothetical particle, it has placed constraints on their characteristics, which will aid future research.
“The sensitivity of a dark matter hidden photon experiment depends on the strength of the dark matter signal compared to the smallest signal you can detect,” team member Nikita Klimovich, a research fellow in the University of Oxford’s Department of Physics, told Phys.org.
“When searching for hidden photons, the amplitude of the dark matter signal depends on the area of the metal antenna used, while the minimum detectable signal level is largely determined by the noise level of the amplifiers used to read the signal from the antenna,” Klimovich added.
The team’s search for dark photons was inspired by a previous attempt to search for hidden dark matter, the SHUKET experiment, which used an electromagnetic telescope. Previous studies that inspired this work, such as the SHUKET experiment, generally aimed to maximize signal power by using a very large antenna with the best commercially available low-noise amplifiers.
However, in the new study, the team took a different approach, using quantum-confined amplifiers instead of standard ones and hunting for dark photons at incredibly low temperatures. They studied them at temperatures ranging from minus 459 degrees Fahrenheit (minus 272.9 degrees Celsius) to minus 459.6682 degrees Fahrenheit (minus 273.149 degrees Celsius), which is just a fraction of a degree above the lowest theoretically possible temperature—absolute zero.
Although this allowed the scientists to significantly reduce the minimum detectable signal levels compared to other experiments using off-the-shelf equipment, this method had a serious drawback. The small volume of the cryostat’s vacuum insulation, which the scientists used to cool their apparatus, severely limited the size of the spherical metal bowl they could use in their research.
While this meant a much weaker signal than that detected by SHUKET and other dark matter experiments, the team hoped that this shortcoming would be offset by the increased sensitivity of the measurements they collected.
“If there were a hidden photon with a mass corresponding to the frequency range we’re sensitive to, we would see a slight excess of power coming from the antenna compared to the reference,” Klimovich said. “Since we didn’t observe such a signal, we could set a new upper limit on the interaction of such a hidden photon particle with an electromagnetic field, based on the smallest signal level we could detect.”
Although the dark photon signal was not present in the team’s measurements, the approach taken by the scientists placed new, stringent constraints on the theoretically hidden photons. As the search for dark matter candidates continues, these constraints and the new approach may ultimately play a role in the discovery of dark photons and, thus, in solving the mystery of dark matter.
Some scientists speculate that dark matter may consist of tiny black holes formed by the collapse of so-called “dark baryons.” Alternatively, dark matter may be a type of particle created by Hawking radiation at the cosmic horizon.
For decades, WIMPs (weakly interacting massive particles) were considered the leading candidate for dark matter. But as the search for WIMPs hits a dead end and experiments continue to fail, new theories of dark matter are emerging. Among them are two new models developed by Stefano Profumo, a professor of theoretical physics at the University of California, San Diego, whose ideas offer a completely different perspective on the dark matter problem.
In one of his papers, Profumo considers whether a “dark sector” could be the source of dark matter. By “dark sector,” he doesn’t mean that our universe is governed by dark matter and dark energy. Instead, he means a kind of “mirror world” of particles interacting through forces beyond the control of matter in our world. Profumo’s dark sector and black hole hypothesis was published on May 9, 2024, in Physical Review D, and his cosmic horizon model was published in the same journal on July 8.
Profumo argues that this concept isn’t as strange as it might seem. For example, he emphasizes how the quarks inside protons and neutrons are bound together by the strong nuclear force. Electrons are completely insensitive to the strong force. They don’t feel it at all. For them, the strong force is a dark realm.
A diagram of the history of the Universe and the location of the cosmic horizon. Quantum effects at the horizon could have led to the emergence of dark matter in the early Universe. Stefano Profumo
According to Profumo, dark baryons would be equivalent to protons or neutrons in this dark sector, except that they could contain more than three quarks and therefore be more massive.
The next step in the researcher’s theory was inspired by his teenage son, who wondered whether a sufficiently massive particle could collapse under its own gravity, forming a mini-black hole. Dark baryons in the dark sector, if it exists, could be massive enough to do just that—and these tiny black holes could be common throughout the universe and collectively form what we call dark matter.
Black holes, large or small, are surrounded by an invisible boundary called the event horizon, within which gravity is so strong that even light cannot escape. However, the event horizon is “hot”—particles created at the boundary by quantum effects can emit what we call Hawking radiation (named after the renowned physicist Stephen Hawking, who is credited with the idea). Over time, Hawking radiation strips mass and energy from the black hole, causing it to gradually evaporate. For supermassive black holes, this would take an unimaginably long time—at least 10^100 years. However, black holes on the smallest scales—which we call the Planck scale—can evaporate in a time shorter than the age of the universe.
However, if certain assumptions are made about the nature of these black holes formed by the collapse of dark baryons, then their Hawking radiation can be suppressed, preventing them from evaporating and allowing them to act as dark matter.
Meanwhile, another of Profumo’s ideas also uses the concept of Hawking radiation, but in a completely new way.
We live in a universe that is expanding at an accelerating rate, carrying with it regions of space so distant that their light will never reach us. This leads to a boundary, or cosmic horizon, that defines the edge of the visible universe. Beyond this horizon, there may be much more of the universe, but we will never see it.
Now let’s return to the Big Bang. The universe began with a burst of expansionary energy known as inflation. This period of inflation lasted a tiny fraction of a second. However, some models also suggest that after inflation, a second, short burst of expansionary energy occurred. Essentially, this period of mini-inflation could be related to inflation and its end, or could be caused by similar factors.
This second period of expansion created cosmic horizons similar to the one that borders the visible universe today. However, the visible universe today has a diameter of 93 billion light-years, with Earth at its center (the concept of a “visible” universe is highly observer-dependent—observers in different parts of space will see different volumes of the observable universe centered around them). The vast size of the visible universe means that the temperature at the cosmic horizon is very low, as space itself has become highly distributed.
This composite image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed by the violent collision of massive galaxy clusters. Blue areas indicate the location of most of the cluster’s mass, which is dominated by dark matter. NASA, ESA, CFHT, CXO, MJ Jee, A. Mahdavi
However, during the second burst of inflation, the Universe was still incredibly compact, and the temperature at the horizon was still extremely high.
Profumo realized that these early cosmic horizons might act like event horizons; indeed, the concept is somewhat reminiscent of a black hole, but inverted, since everything beyond the cosmic horizon is forever separated from us, just as everything within a black hole’s event horizon is separated from us. And just as Hawking radiation is emitted by a black hole’s event horizon, Profumo hypothesized that the cosmic horizon might also experience Hawking radiation, and that the energy from this radiation might be converted into some kind of dark matter particle.
This may seem somewhat arbitrary, since the position of the cosmic horizon depends on the observer’s location. However, since the universe is homogeneous (the same at every point on large scales) and isotropic (the same in all directions)—two truisms we call the cosmological principle—any two observers should see the same amount of dark matter, no matter where they are.
Profumo does not claim that dark matter must necessarily be one of these two possibilities; in fact, the fact that he has developed two theories implies that he does not wish to tie his colours to any particular theory.
All we know for sure about dark matter is that it interacts via gravity, and yet, despite its enigma, it completely dominates how matter in the universe assembles into galaxies. Nearly a century has passed since Fritz Zwicky first proposed its existence, and about half a century since Vera Rubin confirmed the necessity of dark matter in our universe, but we still know nothing about it. Experiments can narrow down its properties, so the more ideas we have, the more likely we are to match one of them with the observed properties of dark matter.
Some researchers have suggested that dark matter doesn’t exist at all—perhaps our understanding of gravity breaks down at large scales. One of the most prominent alternative theories is called Modified Newtonian Dynamics (MOND). It corrects for the behavior of gravity at very low accelerations, such as at the outskirts of galaxies.
However, the MOND theory and similar theories cannot explain all the available data, especially the detailed patterns in the cosmic microwave background and gravitational lensing. This is why most scientists still believe dark matter is a real substance that we simply cannot yet directly detect.
Dark matter doesn’t interact with light, so we can’t see it directly. Instead, scientists detect its presence through its gravitational influence on visible objects.
Cosmologists generally believe the answer is that dark matter explains the behavior of galaxies. This is partly because it has been so difficult to formulate a successful theory of MOND (modified gravity). And partly because when scientists pointed microwave telescopes at the cosmic background radiation (CMB), the light from the early universe, it turned out that, according to general relativity, the same amount and type of dark matter was also required to explain the behavior of sound waves that propagated through the universe when it was less than 500,000 years old, and whose fingerprints we can see. Modified gravity struggles to provide a unified explanation for all these systems—galaxies, galaxy clusters, the universe itself.
If dark matter exists, it must have mass. Massless dark matter could not behave in a way that would solve the problems that dark matter does.
Two things we know for sure about dark matter (assuming it exists) are that it exerts a gravitational force (has mass) and that it moves slowly (compared to the speed of light).
Experiments, often conducted deep underground to protect against cosmic rays, are designed to detect dark matter particles interacting with the detector. Experiments like LUX-ZEPLIN (LZ) and XENONnT take sensitivity to a new level.
The Large Hadron Collider (LHC) collides particles at high energies, potentially producing dark matter particles that are undetectable but leave missing energy and momentum in the collision data.
Telescopes around the world and in space continue to map the distribution of dark matter using gravitational lensing and galaxy surveys. Instruments like the Vera C. Rubin Observatory and the European Space Agency’s Euclid mission will provide even better data.
Specialized experiments such as ADMX (the Axion Dark Matter Experiment) and sterile neutrino searches are testing other potential dark matter candidates.
More information about dark matter can be found on the website of the Fermi National Accelerator Laboratory (USA), which conducts high-energy particle experiments at cutting-edge particle colliders with the goal of discovering particles that could fill gaps in our understanding of the universe. The European Organization for Nuclear Research (CERN), the world’s largest particle physics laboratory, is also searching for missing dark matter particles. NASA, Dark Energy, Dark Matter, https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy, CERN, Dark Matter, https://home.cern/science/physics/dark-matter
Dark matter accounts for approximately 80% of the universe’s mass. MARK GARLICK/SCIENCE PHOTO LIBRARY
The antigravity force that drives the accelerating expansion of the universe appears to be distributed uniformly throughout space and time.
According to the research team, the results also allow for a more precise determination of the proportion of energy and matter in the Universe that is dark energy. The team’s study was published in April 2023 in the journal Monthly Notices of the Royal Astronomical Society.
They reached their conclusions after analyzing observations of galaxy clusters made by the eROSITA X-ray telescope, which scans the entire sky above Earth in search of detectable galaxy clusters. eROSITA is mounted on Spektr-RG, a Russian-German space telescope launched into Earth orbit in 2019.
Galaxy clusters are useful for understanding dark energy, since on large scales, this strange, repulsive “antigravity” force should suppress the formation of gigantic cosmic structures. This means that dark energy determines how and where galaxy clusters, the largest objects in the universe, can form.
The eROSITA Equatorial Final Depth Survey (eFEDS) discovered approximately 500 galaxy clusters, one of the largest samples of low-mass galaxy clusters discovered to date. The observed clusters span approximately the last 10 billion years of the evolution of the 13.8-billion-year-old Universe.
X-ray (top) and optical false-color (bottom) images of three low-mass galaxy clusters studied by the eROSITA telescope. The cluster with the highest redshift dates back to a period when the Universe was approximately 10 billion years younger than today. In this case, the cluster galaxies are clearly significantly redder than the galaxies in the other two clusters. eROSITA
The research team combined eROSITA observations with optical data obtained during Subaru’s Hyper Suprime-Cam strategic mission. This enabled the first cosmological study using galaxy clusters discovered by eROSITA.
The study’s results were compared with theoretical predictions, confirming that dark energy accounts for approximately 76% of the total energy density of the Universe. The results also indicate that this energy density is uniform in space and constant over time.
The team’s results are consistent with those of other independent approaches to studying dark energy, such as previous studies of galaxy clusters and research based on the effect of gravity on light, known as weak gravitational lensing. However, while these new discoveries shed more light on dark energy, the force remains a mystery that physicists are eager to unravel.
In the 1920s, American astronomer Edwin Hubble observed distant galaxies, which revealed that they were moving away from us. Moreover, the farther away a galaxy is, the faster it is moving away, leading scientists to discover that the universe is expanding.
This was shocking enough to challenge the then-prevalent notion that the universe existed in a stable state. The situation became even stranger in 1998, when observations of distant supernovae revealed that the universe was not only expanding but also accelerating.
To explain this acceleration, scientists need a source, and they call this source “dark energy,” which provides a kind of “antigravity” to accelerate the cosmic expansion.
However, despite knowing what dark energy does and being able to calculate that it makes up about 76% of the energy and matter in the Universe, scientists still don’t know what it actually is or why it began to affect the Universe in its later epochs.
