Dark matter remains elusive despite the prolonged of the world’s greatest minds.
It continues to be sought as it provides the missing link to several theories and beliefs. While it is believed to make around 25% of all energy in the universe, it has never be detected.
Dark matter will not either emit or absorb light. It won’t interact with three of the four primary forces of nature. These mysterious characteristics make it nearly impossible to observe or detect.
Researchers have conducted countless studies in an attempt to unravel the mysteries of dark matter. Some scientists believe it can only be found in a hidden dimension, while others believe it exists in neutron stars or black holes.
Despite the setbacks, most astrophysicists still firmly believe that dark matter exists. This is because several cosmic activities, like the rotation of galaxies, can’t be explained using traditional physics without another form of matter.
Gravitino
The gravitino hypothesis represents a massive dive into the world of theoretical physics. During the 1960s and 1970s, physicists came up with the theory of supersymmetry to account for the gaps left behind in the Standard Model of particle physics.
Supersymmetry believes that there needs to be a theoretical counterpart within each particle of the Standard model (i.e. photon, electron, Higgs). These opposing particles have similar properties as the originals, with the exception of a few fundamental differences in their angular momentum.
Another separate theory claims a graviton, which is a particle without mass that controls the force of gravity, similar to how the photon mediates electromagnetism. The gravitino is the result of tying these two different theories together. Thus, the gravitino becomes the gravitron’s hypothetical supersymmetric partner that some scientists believe could make up dark matter.
Self-Interacting Dark Matter
Perhaps the biggest annoyance with dark matter is that it rarely ever obeys the predictions of scientists. According to computer simulations, this matter should materialize itself into what is called a “cusp distribution.”
This theory claims that dark matter will exist at the epicenter of a given galaxy; some will be concentrated in a highly dense sphere, while the remaining substance would exist as a lingering vapor.
However, In reality, physicists have determined that dark matter appears to behave oppositely. It is now believed that dark matter orbits around the galaxy’s edge, composed of a stand-off halo structure. This configuration is called the “core distribution.” And because of this, we have a “cusp-core” issue.
To describe this cusp-core discrepancy, physicists created a theory of self-interacting dark matter. This model claims that dark matter particles must interact using forces that are unknown to traditional physics.
However, this theory is controversial, and not all parties are on board. The heating of dark matter is another theory that believes dark matter gets propelled from a galaxy’s center when new stars are formed.
Weakly Interactive Massive Particle (WIMP)
For many years, the biggest dark matter candidate was the famous weakly interacting massive particle or WIMP for short. The WIMP model was created in the 1970s to expand upon the existing 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 an extensive array of experiments, none of the attempts to find WIMPs have been successful. It was initially believed Geneva’s LHC would reveal their existence. But after a decade, it has yielded no such evidence of that. Similarly, the extremely sensitive tanks of liquid xenon that have been buried deep in South Dakota haven’t come up with anything either.
With physicists constantly failing to detect any particles directly, there is some serious doubt about the WIMP hypotheses.
Axion
Axions are slow-moving particles that are believed to be neutrally charged. They have an extremely light mass that is some billion times lighter than electrons. They have a weak interaction with other matter and light. Because of these unique properties, it gives cosmologists the flexibility to include them in theories regarding dark matter. However, these properties make them extremely difficult to find as well.
Only axions within a small range of masses would be able to make up dark matter. If they were to be much heavier or lighter, their observations would have already been made by now. This tiny window of possibility could make it easier to confirm or rule out the axion hypothesis than other dark matter theories.
In April 2018, the detection of axions was attempted at the University of Washington, as they began an Axion Dark Matter Experiment (ADMX). The theory states that whenever axions pass through a magnetic field, they may decay spontaneously into two photons –individual packets of light.
Thus, if axions from outer space are constantly flowing around the Earth unnoticed, then the ADMX’s mighty magnet should be able to transform a few of them into microwave photons. So far, no evidence has been recorded.
Kaluza-Klein Particles
Popular convention says that our universe resides within four dimensions—three of those dimensions are spatial, and the remaining dimension is time. However, in recent years, scientists wonder if there could be even more dimensions.
Beginning with Einstein’s infamous theory of general relativity, theoreticians Theodor Kaluza and Oskar Klein postulated a fifth dimension hidden and arched across the galaxy. This model was initially published in 1921. It includes an assortment of hypothetical particles, and the lightest of these particles is a potential candidate for dark matter.
Because of their nature to interact, the Kaluza-Klein (KK) particles are a few candidates that could be directly detected. Also, two KK particles annihilate one another when they crash together.
During these crashes, particles that are similar to neutrinos and photons are blasted out of the collision. This means they could be detected because of the unique energy patterns they create. The high-energy LHC is searching for evidence of KK particles and an extra dimension. But none have been reported.
Sterile Neutrinos
The research and study of neutrinos have become one of the more fascinating areas of contemporary physics. In 2015, Arthur B. McDonald and Takaaki Kajita won the Nobel Prize in Physics for demonstrating how neutrinos periodically change “flavor” as they travel across the galaxy.
At present, only three known neutrino flavors exist—muon, electron, and tau. Each of these moves too quickly to exist in dark matter. However, scientists at Fermilab have considered that a fourth flavor may exist and could be a potential dark matter candidate. They are calling it the sterile neutrino.
Within their MiniBooNE experiment, they are scouring through particle beams as they search for this fourth neutrino flavor. The detector is comprised of a large round tank that contains over 800 tons of mineral oil.
In 2018, MiniBooNE reported some promising results that insinuated the potential existence of sterile neutrinos. But results from the MINOS+ experiment in 2019 contradicted that 2018 study.
