British scientists believe that an entirely new fundamental particle might hold the key to unraveling the mystery of “dark matter,” which is thought to make up a significant portion of the universe’s mass.
What is dark matter?
In astrophysics, the term “dark matter” refers to a hypothetical form of matter in the universe with components not yet understood. Dark matter doesn’t emit or reflect enough electromagnetic radiation to be observed through current telescopes or measuring devices. However, its presence can be inferred through its gravitational effects on solid matter and/or other celestial objects, as well as on the entire universe. Based on current knowledge of larger cosmic structures like galaxies and widely accepted theories of the Big Bang, scientists believe that dark matter is a fundamental component, accounting for up to 70% of the universe’s total mass (dark matter + ordinary matter).
Scientists estimate that dark matter makes up 26.8% of the universe’s combined energy-mass, while ordinary matter accounts for only 4.9%. In terms of mass alone, dark matter is believed to constitute a whopping 84.5% of the universe.
For astronomers, dark matter reveals its traces through the gravitational forces it exerts on stars and galaxies, allowing them to connect these celestial bodies and establish the structure of the universe. Traces of dark matter can also be observed in the cosmic microwave background (CMB), which is the remnant radiation from the Big Bang. However, despite countless research efforts, no one has directly observed dark matter to date.
Who discovered dark matter? In the late 19th century, astronomers began to speculate about unseen matter, which included stars that were gradually fading away or interstellar gas and dust. Some researchers even started estimating the mass of this mysterious matter. Most of them believed that this enigmatic matter constituted only a small fraction of the total mass of the universe.
It wasn’t until 1933 that Swiss-American astronomer Fritz Zwicky noticed that distant galaxies were rotating much faster than they should be based on the observable matter through telescopes. He suggested, “If this is confirmed, we will have a surprising result that dark matter is much more abundant than visible matter.”
However, many researchers in the field remained skeptical of Zwicky’s findings until the 1970s when astronomers Kent Ford and Vera Rubin conducted detailed studies on stars in the outer regions of our neighboring Andromeda Galaxy. These stars were orbiting the galaxy’s core at an extremely high speed, as though they were influenced by an unseen matter with gravitational pull, which later scientists observed happening in all galaxies throughout the universe.
Researchers still don’t know what this invisible mass contains. Some speculate that dark matter is composed of small black holes or other solid objects that emit very little light and are seldom visible through telescopes. In the 1990s, observational results became even more intriguing when the Wilkinson Microwave Anisotropy Probe (WMAP) showed that dark matter could be up to five times heavier than visible matter.
Why is dark matter still a mystery?
Observations through telescopes seem to never find enough small solid objects to account for the widespread phenomenon of dark matter. Most astronomers today believe that dark matter consists of subatomic particles with properties significantly different from the protons and neutrons we are familiar with.
The most prominent “heavyweight” candidate for dark matter is called Weakly Interacting Massive Particles (WIMPs). These speculative entities are not part of the Standard Model of particle physics, which describes most known particles and forces. WIMPs may resemble ghostly neutrinos but are much heavier, about 10 to 100 times the mass of a proton. (The exact mass of neutrinos is still undetermined, but they are much lighter than electrons.)
Like neutrinos, WIMPs only interact through two of the four fundamental forces in the universe, namely gravity and the weak nuclear force. These dark matter particles do not interact with electromagnetism, the basis of light, and are thus perpetually invisible.
Physicists have constructed massive detectors and placed them deep underground to shield them from cosmic rays in an effort to search for WIMPs, but so far, no experiment has provided evidence of their existence. In recent years, this failure has led some researchers to question whether we are chasing after a type of particle in nature that may never be found.
As a result, some scientists have shifted their focus to a newer dark matter candidate known as the axion, a particle much smaller by millions or even billions of times compared to an electron. These hypothesized particles are particularly appealing to researchers because they may also address another challenging issue in physics: explaining why they can interact with neutrons to sense gravity but not with electromagnetic fields.
In June 2020, scientists at the Gran Sasso National Laboratory in Italy announced that they had accidentally stumbled upon a small signal that could be explained by axions. These findings shocked the scientific community but have not been confirmed by other experiments.
Is dark matter real? When this question is raised, researchers are still scratching their heads to answer what dark matter is. Some theorists wonder if there is an entirely dark component in the universe, with numerous types of particles and even dark forces that only affect dark matter, mirroring the complexity of the visible atomic world.
At the same time, a minority of scientists believe that dark matter is an illusion. They propose an idea called Modified Newtonian Dynamics (MOND), a hypothesis suggesting that on large scales, gravity behaves differently from what we think, which would explain the rotational behavior of stars and galaxies. However, most experts do not see the need to start with such a departure from conventional physics, as it would require a significant shift in our understanding of the majority of the well-established realities.
Dark matter is unrelated to dark energy, another mysterious phenomenon causing the universe to expand rapidly. These terms merely share the word “dark,” which scientists often use to refer to phenomena they do not fully understand.