The enigmatic matter does not absorb or emit light. It also doesn’t interact with the three of the four fundamental forces of nature. These elusive properties make it almost impossible to pin down. Researchers across the globe are itching to uncover the mysteries of dark matter—from the search for WIMPs at the Large Hadron Collider (LHC) to the University of Washington’s cutting-edge axion detector. While some theories predict the answer will be found in an extra hidden dimension, others prefer black holes and neutron stars. Despite the lack of direct evidence, the vast majority of astrophysicists still believe that dark matter is out there. Cosmic phenomena like the rotation of galaxies cannot be explained through traditional physics unless a hidden form of matter is present.
10 Weakly Interactive Massive Particle (WIMP)
For decades, the most popular candidate for dark matter has been the weakly interacting massive particle (WIMP). The hypothetical particle was first dreamed up in the 1970s as an expansion of the traditional Standard Model of particle physics. The theory is that the cosmos is swarming with invisible, neutrally charged particles that came into being shortly after the big bang. The idea of invisible particles is nothing particularly new. Scientists are already aware of the neutrino—the difficult-to-detect subatomic particle that races across galaxies with a mass fractionally above zero. In comparison, WIMPs are believed to be much heavier and more sluggish, trudging across the sky in dense clumps and intricate structures. That is, if they even exist at all. Despite a large array of experiments, none of the attempts to find WIMPs have been successful. It was originally thought that the LHC in Geneva would be able to shed light on their existence. But almost a decade after it opened, no evidence has been found. Similarly, the highly sensitive tanks of liquid xenon buried deep under South Dakota discovered nothing in their search, either.[1] With scientists continually failing to detect these particles directly, hypotheses surrounding WIMPs are now cast in serious doubt. One astrophysicist writing for Forbes Magazine compared the persistent search to a “drunk looking for his lost keys beneath the lamppost.” It would be an oversight to rule out WIMPs altogether. But it looks like scientists have to return to the drawing board and consider alternative theories of dark matter as well.
9 Massive Astrophysical Compact Halo Object (MACHO)
Another less exotic explanation for dark matter is the existence of massive astrophysical compact halo objects (MACHOs). These include black holes, neutron stars, and brown dwarfs—ultracompact stellar objects composed of regular matter. MACHOs cannot be detected using typical methods because they emit little or no radiation. Instead, these muted giants are observed by studying light from distant stars through a process known as microlensing. Due to their immense mass, MACHOs bend and focus rays of light around themselves, which causes the rays to appear brighter. The level of distortion depends on the mass of the MACHO. By observing the light, scientists are able to calculate the amount of hidden matter present. However, not enough MACHOs have been found lurking around to account for all the dark matter in the universe. As such, the search for another candidate continues.[2]
8 Axion
Axions are predicted to be neutrally charged, slow-moving particles with a mass around a billion times lighter than an electron. Their interaction with light and other matter is relatively weak, which gives cosmologists confidence in their potential to make up dark matter. But it also makes them incredibly difficult to detect. Only axions from a narrow range of masses are able to constitute dark matter. If they were much lighter or heavier, observations would have been made by now. This limited window of possibility means that the task of ruling the axion hypothesis in or out is relatively simple when compared to other candidates. The latest attempt to detect axions began in April 2018 when astrophysicists at the University of Washington launched their Axion Dark Matter Experiment (ADMX). According to the theory, when axions pass through a magnetic field, they could be able to decay spontaneously into two photons (individual packets of light). If axions from the Milky Way are constantly whizzing through the Earth unnoticed, then the ADMX’s highly powerful magnet would convert some of them into microwave photons. An incredibly sensitive detector is in place to pick up any photons produced, but so far, no evidence has been reported.[3]
7 Gravitino
The gravitino hypothesis delves deep into the realms of theoretical physics. In the 1960s and 1970s, scientists developed the theory of supersymmetry to explain some of the gaps left by the Standard Model of particle physics. Supersymmetry predicts that for each particle in the Standard model (e.g., electron, photon, Higgs), there should be a theoretical counterpart. These partner particles share similar properties to the originals except for some fundamental differences in their intrinsic angular momentum. A separate theory predicts the existence of the graviton—a massless particle that mediates the force of gravity, similar to the photon mediating electromagnetism. Tying these two theories together is the gravitino—the hypothetical supersymmetric partner to the graviton that some physicists believe could constitute dark matter.[4]
6 Kaluza-Klein Particles
Our universe is said to be comprised of four dimensions—three spatial dimensions plus time. However, for the last century, scientists have pondered whether more could exist. Expanding Einstein’s groundbreaking theory of general relativity, theoreticians Theodor Kaluza and Oskar Klein predicted a hidden fifth dimension arching across the universe. First published in 1921, their model includes an array of hypothetical particles, the lightest of which is a possible candidate for dark matter. Due to their interactive nature, the Kaluza-Klein (KK) particles are among only a handful of candidates that could be detected directly. Furthermore, when two KK particles come crashing together, they annihilate each other. In the melee, particles like photons and neutrinos get fired out. They can be detected due to their distinctive energy patterns. The high-energy LHC continues to search for evidence of an extra dimension and KK particles. But so far, none has been reported.[5]
5 Fuzzy Dark Matter
Fuzzy dark matter is a relative newcomer to the lineup of dark matter candidates. The theory first began to find traction around the turn of the century. Before that, only a few pockets of physicists were interested, and even then, they barely communicated with each other. As such, fuzzy dark matter goes by several different names, each independently put forward by a different research team. Scalar field dark matter, ultra-light axion-like particle, wave dark matter, fluid dark matter, and repulsive dark matter are just a few. Despite the plethora of names, the theories are all roughly the same. They postulate that dark matter is formed from an immense number of tiny particles with exceptionally low mass. At incredibly cold temperatures, the particles coalesce to form a bizarre type of matter known as a Bose-Einstein condensate. In condensate form, these particles have almost no energy and behave like one cohesive body. The individual particles have almost no effects on their surroundings. En masse, however, they can distort rays of interstellar light. The amount of distortion depends on the mass of the dark matter particles. Therefore, scientists are able to search for fuzzy dark matter by examining archive data from observatories like the Very Long Baseline Array in New Mexico.[6]
4 Self-Interacting Dark Matter
One of the key frustrations around dark matter is that it refuses to obey scientists’ predictions. According to computer-generated models, the substance should structure itself into something known as the “cusp distribution.” This theory predicts that dark matter can be found at the heart of a galaxy, some of it concentrated in a dense sphere and the rest lingering around as a vapor. In reality, cosmologists have observed that dark matter behaves in almost the opposite way: It orbits around the edge of a galaxy in a far-off halo structure. This has been named the “core distribution.” From it, the “cusp-core” problem arises.[7] To explain the cusp-core discrepancies, scientists came up with the theory of self-interacting dark matter. This model proposes that, because it is so mysterious and difficult to understand, dark matter particles interact with each other through forces that physics is currently unable to explain. However, not everyone is on board with this explanation. Another theory—dark matter heating—suggests that dark matter is propelled from the center of a galaxy by energy and wind created during the formation of stars.
3 Sterile Neutrinos
Neutrino research is one of the most fascinating areas of contemporary physics. In 2015, Takaaki Kajita and Arthur B. McDonald were awarded the Nobel Prize in Physics for demonstrating that neutrinos periodically change “flavor” on their journey across the universe. Presently, there are only three known “flavors” of neutrino—electron, muon, and tau. All of them are far too speedy to make up dark matter. However, researchers at Fermilab in Illinois are pursuing a fourth flavor and potential dark matter candidate: the sterile neutrino. Their MiniBooNE experiment scours through intense beams of particles in search of the elusive fourth flavor. The detector consists of a large spherical tank filled with over 800 tons of mineral oil. In 2018, MiniBooNE produced promising results that hint at the existence of sterile neutrinos.[8] However, the MINOS+ experiment results reported in 2019 contradicted the 2018 study. Obviously, there is no consensus yet.
2 Dark Photons
As discussed previously, a photon acts as a single particle of light and mediates the electromagnetic force, one of the fundamental forces of nature. To explain the conundrum of dark matter, some experts have proposed the idea of dark photons—hypothetical force mediators similar to regular photons with extremely low mass. In fact, some researchers believe that gravitational waves—celestial ripples in the fabric of space and time—could be the key to uncovering these miniscule particles. If dark photons are skulking around the universe, their distinctive signals could be picked up by highly sensitive gravitational wave detectors like LIGO and Virgo. As scientists eagerly await the launch of the Laser Interferometer Space Antenna (LISA)—the first space-based gravitational wave observatory—it seems that we are one step closer to finally pinning down dark matter.[9]
1 Dark Matter Does Not Exist
As time rolls on, the lack of evidence for any of the candidates is causing some physicists to wonder if they have made a mistake. Perhaps dark matter does not exist at all. Maybe there is another explanation after all. One of the most prominent dark matter skeptics is Israeli physicist Mordehai Milgrom, who first proposed his rival theory of Modified Newtonian Dynamics (MOND) in the 1980s. In his maverick paper, Milgrom argues that the traditional physics laid out by Isaac Newton begins to fall apart on an extremely large scale. If this is true, it completely alters current ideas about stars in the outer reaches of a galaxy. Under MOND, dark matter is not necessary to explain their unusual motion.[10] So, is dark matter an enormous blunder? This would not be the first time that physicists have made a mistake on such a large scale. During the 19th century, there was a widely held belief that our universe was brimming with an invisible substance known as luminiferous ether. For decades, it was thought that ether was needed for rays of light to propagate. Then the pivotal Michelson-Morley experiment of 1887 essentially disproved its existence. In reference to this, Milgrom has described dark matter as “our generation’s ether.” Whether dark matter exists and in what form remains one of the great mysteries of modern science. Future evidence might show that all the theories listed here are completely wrong. On the other hand, we could be within a hair’s breadth of a major breakthrough. With every new dark matter detector and every null result, we edge closer to finding the truth.
