Buzz
Despite extensive experiments and insights from some of the brightest minds in science, the search for dark matter persists. It could account for a significant portion of the universe's energy density, yet all direct detection efforts have so far failed.
This mysterious substance neither absorbs nor emits light and doesn't interact with three of the four fundamental forces of nature. These peculiar characteristics make it nearly impossible to identify.
Scientists around the world are eager to unravel the secrets of dark matter—ranging from the hunt for WIMPs at the Large Hadron Collider (LHC) to the University of Washington’s innovative axion detector. While some theories suggest the answer lies in a hidden extra dimension, others point to black holes and neutron stars.
Although no direct evidence has been found, most astrophysicists remain convinced that dark matter exists. Phenomena like galaxy rotation can't be explained by conventional physics unless there is some form of unseen matter at play.
10. Weakly Interactive Massive Particle (WIMP)
For many years, the prime contender for dark matter has been the weakly interacting massive particle (WIMP). This theoretical particle was conceived in the 1970s as an extension of the Standard Model of particle physics. The hypothesis suggests that the universe is filled with invisible, electrically neutral particles that originated just after the big bang.
The concept of invisible particles is not entirely novel. Scientists are already familiar with the neutrino—a notoriously elusive subatomic particle that travels through galaxies with a mass just slightly above zero. In contrast, WIMPs are thought to be much more massive and sluggish, moving through the universe in dense clusters and complex structures. That is, if they exist at all.
Despite numerous experiments, all attempts to detect WIMPs have been unsuccessful. Initially, it was believed that the LHC in Geneva would provide insights into their existence, but nearly a decade after its activation, no evidence has emerged. Similarly, the highly sensitive liquid xenon tanks buried deep in South Dakota found nothing in their search either.
As scientists continue to struggle with detecting these particles directly, doubts are beginning to surface about the validity of WIMP theories. One astrophysicist, writing for Forbes Magazine, likened the ongoing search to a “drunk looking for his lost keys beneath the lamppost.”
It would be premature to dismiss WIMPs entirely. However, it seems scientists must now rethink their approach and explore alternative dark matter theories.
9. Massive Astrophysical Compact Halo Object (MACHO)
Another plausible explanation for dark matter is the existence of massive astrophysical compact halo objects (MACHOs). These include black holes, neutron stars, and brown dwarfs—compact stellar bodies made of ordinary matter. MACHOs are not detectable through conventional means as they emit very little or no radiation at all.
Instead, these elusive giants are detected by observing light from distant stars through a technique called microlensing. Due to their tremendous mass, MACHOs distort and amplify light passing nearby, making the light appear brighter.
The degree of distortion depends on the mass of the MACHO. By analyzing the light, scientists can estimate the amount of hidden matter present. However, not enough MACHOs have been discovered to account for all the dark matter in the universe. As a result, the search for other potential candidates continues.
8. Axion
Axions are theorized to be neutral, slow-moving particles with a mass roughly a billion times lighter than an electron. Their weak interaction with light and other matter makes them a promising candidate for dark matter. However, this characteristic also makes them extremely challenging to detect.
Only axions within a specific mass range can make up dark matter. If they were significantly lighter or heavier, observations would have already been made. This narrow range of possibilities makes it easier to confirm or disprove the axion hypothesis compared to other dark matter candidates.
The most recent effort to detect axions began in April 2018 when astrophysicists from the University of Washington initiated their Axion Dark Matter Experiment (ADMX). According to the theory, when axions travel through a magnetic field, they may spontaneously decay into two photons (light particles).
If axions from the Milky Way are silently passing through the Earth, the ADMX’s powerful magnet could convert some of them into microwave photons. An exceptionally sensitive detector is set up to capture any photons produced, but so far, no evidence has been detected.
7. Gravitino
The gravitino theory dives into the depths of theoretical physics. In the 1960s and 1970s, scientists introduced the concept of supersymmetry to fill in the gaps left by the Standard Model of particle physics.
Supersymmetry suggests that for every particle in the Standard Model (such as the electron, photon, and Higgs), there should exist a theoretical counterpart. These partner particles possess similar properties to their originals, except for certain fundamental differences in their intrinsic angular momentum.
Another theory proposes the existence of the graviton—a massless particle responsible for mediating gravity, akin to how the photon mediates electromagnetism. Connecting these two theories is the gravitino, the hypothetical supersymmetric partner to the graviton, which some physicists believe may account for dark matter.
6. Kaluza-Klein Particles
Our universe is generally understood to consist of four dimensions—three spatial dimensions plus time. However, for the past century, scientists have wondered whether additional dimensions might exist beyond the ones we observe.
Building on Einstein’s revolutionary theory of general relativity, theorists Theodor Kaluza and Oskar Klein hypothesized the existence of an unseen fifth dimension stretching across the universe. Their 1921 model introduces a range of hypothetical particles, with the lightest among them potentially being a candidate for dark matter.
Thanks to their interactive properties, Kaluza-Klein (KK) particles are one of the few dark matter candidates that could be directly detected. Additionally, when two KK particles collide, they annihilate one another.
In this annihilation, particles such as photons and neutrinos are emitted, and their unique energy signatures can be detected. The high-energy LHC is still on the lookout for evidence of an extra dimension and KK particles, but no such findings have been reported so far.
5. Fuzzy Dark Matter
Fuzzy dark matter is a relatively new addition to the roster of dark matter theories. It gained significant attention around the turn of the 21st century, though prior to that, it was only a small group of physicists showing interest, and they barely exchanged ideas.
As a result, fuzzy dark matter is known by a variety of names, each proposed by a separate research team. These include scalar field dark matter, ultra-light axion-like particles, wave dark matter, fluid dark matter, and repulsive dark matter, among others.
Despite the many names, the core idea remains the same: dark matter is thought to consist of a vast number of minuscule particles with extraordinarily low mass. When these particles are cooled to near absolute zero, they form a strange state of matter known as a Bose-Einstein condensate, where they behave like a unified whole with almost no energy.
The individual particles barely influence their environment. However, in large quantities, they can bend light traveling through space. The extent of this distortion is linked to the mass of the dark matter particles. As a result, scientists are using data from observatories like the Very Long Baseline Array in New Mexico to search for signs of fuzzy dark matter.
4. Self-Interacting Dark Matter
One of the major challenges with dark matter is its refusal to follow the predictions made by scientists. Computer models suggest that it should organize itself into a structure known as the “cusp distribution,” where dark matter is concentrated in a dense sphere at the center of a galaxy, with the remaining particles floating around as a vapor.
However, observations have revealed that dark matter actually behaves in a way that contradicts this theory. Instead of forming a central mass, dark matter orbits around the outskirts of a galaxy, forming a distant halo. This has led to the creation of the “core distribution” theory, which has given rise to the “cusp-core” problem.
To account for these discrepancies, scientists proposed the theory of self-interacting dark matter. This hypothesis suggests that dark matter particles interact with one another through forces that current physics is unable to fully explain, making it even more mysterious.
However, not all scientists agree with this explanation. An alternative theory—dark matter heating—argues that dark matter is pushed away from the center of a galaxy due to energy and winds generated during star formation.
3. Dark Matter Might Not Exist
As time progresses, the ongoing absence of solid evidence for any of the dark matter candidates is causing some physicists to question whether they've made a fundamental mistake. Could it be that dark matter isn't real after all? Perhaps there's a completely different explanation waiting to be discovered.
Among the leading skeptics of dark matter is Israeli physicist Mordehai Milgrom, who introduced his alternative theory, Modified Newtonian Dynamics (MOND), back in the 1980s. In his unconventional paper, Milgrom argues that Newton's laws of physics begin to break down at an extremely large scale, which could offer a new perspective on the universe.
If Milgrom's theory holds, it could radically change our understanding of stars on the outskirts of a galaxy. Under the principles of MOND, there would be no need for dark matter to account for the strange motion of these stars.
So, could dark matter be an enormous mistake?
This wouldn't be the first instance where physicists have made a colossal error. Back in the 19th century, there was a widespread belief in a mysterious substance known as luminiferous ether that supposedly filled the universe.
For years, it was assumed that ether was necessary for light to travel through space. However, the groundbreaking Michelson-Morley experiment of 1887 effectively disproved the existence of ether. Milgrom has since compared dark matter to “our generation’s ether.”
The question of whether dark matter exists, and in what form, is one of the greatest unsolved puzzles in modern science. Future discoveries might reveal that all of the theories we have today are completely off the mark.
On the other hand, we could be on the verge of a major breakthrough. With each new dark matter detector and each negative result, we move closer to uncovering the truth.
2. Dark Photons
As mentioned earlier, a photon is a single particle of light that mediates the electromagnetic force, one of nature's fundamental forces. To address the dark matter mystery, some theorists have suggested the existence of dark photons—hypothetical force carriers similar to regular photons but with an extremely small mass.
In fact, some scientists believe that gravitational waves—ripples in the fabric of space and time—might hold the key to detecting these tiny particles. If dark photons are indeed present in the universe, their unique signals could be detected by highly sensitive gravitational wave detectors such as LIGO and Virgo.
With the upcoming launch of the Laser Interferometer Space Antenna (LISA)—the first space-based gravitational wave observatory—scientists are getting closer to finally identifying dark matter.
1. Sterile Neutrinos
Neutrino research is one of the most exciting frontiers in modern physics. In 2015, Takaaki Kajita and Arthur B. McDonald were awarded the Nobel Prize in Physics for proving that neutrinos change “flavor” as they travel across the universe.
Currently, only three distinct “flavors” of neutrinos are known—electron, muon, and tau. These are far too fast to account for dark matter. However, researchers at Fermilab in Illinois are investigating a fourth flavor, a potential dark matter candidate: the sterile neutrino.
Their MiniBooNE experiment searches through powerful particle beams in pursuit of the elusive fourth neutrino flavor. The experiment involves a large spherical tank filled with over 800 tons of mineral oil. In 2018, MiniBooNE reported encouraging results that suggest the possible existence of sterile neutrinos. However, conflicting findings from the MINOS+ experiment in 2019 mean there is still no definitive answer.
