In this blog post, we will explore the scientific search for the identity of dark matter, one of the mysteries of the universe, and look at the main particle candidates.
Dark matter, the invisible puzzle of the universe
In the 1930s, while analyzing the rotational speed of spiral galaxies, scientists discovered an anomaly that could not be explained by existing laws of physics. Contrary to Newton’s prediction that the rotational speed of spiral arms should decrease as they move away from the center of the galaxy, it was observed that the rotational speed remained constant. This could not be explained by the existing distribution of mass alone. Most stars are concentrated in the center of the galaxy, so the further away from the center, the slower the rotation speed should be as the orbital radius increases, but this was not the case. To explain this phenomenon, scientists hypothesized the existence of an invisible substance with mass, called “dark matter.”
Dark matter is difficult to observe because it does not emit or reflect light, but it is believed to have gravity and influence the structure and motion of the universe.
Over the decades that followed, various studies were conducted, and recently, the collision of two galaxy clusters was observed, confirming the existence of dark matter more clearly. In particular, advances in particle physics have made it possible to provide theoretical explanations for the nature of dark matter. Particles believed to be dark matter share the characteristics of having mass but interacting with other particles only through gravity or very weakly. As a result, neutrinos, weakly interacting massive particles (WIMPs), and axions have been proposed as leading candidates for dark matter.
Neutrinos – too light to exist
Neutrinos are particles produced when neutrons decay, and are much lighter than protons and electrons. Their exact mass is still unknown, and in the current universe, they move at speeds close to the speed of light. It is estimated that they moved even faster in the early universe, and because of this characteristic, neutrinos have limitations as a major component of dark matter.
According to virtual experiments, dark matter particles must be able to form stable early structures that can become the seeds of galaxies. However, neutrinos are so fast that they disrupt the gravitational centers created by quantum fluctuations in the early universe, preventing the formation of galaxies. Therefore, the prevailing view is that they are not suitable as dark matter.
WIMPs – At the center of dark matter theory
WIMPs are very promising candidates for dark matter predicted by particle physics. These particles interact weakly and are known to have masses typically tens of times greater than that of protons. WIMPs are created in pairs and annihilate in pairs, and in the high-temperature environment of the early universe, they were actively created and annihilated in thermal equilibrium with other particles.
However, as the universe expanded and cooled, the temperature dropped, and the energy needed to create WIMPs disappeared, causing them to annihilate and their density to decrease until their number remained constant. Due to these characteristics, WIMPs move relatively slowly during the formation of the universe and can help form galaxies by gathering around gravitational centers formed by quantum fluctuations. Because they match the characteristics of ideal dark matter in theory, they are also called “WIMP miracles.”
It is assumed that WIMPs exist near Earth, and it is estimated that there is an average of one WIMP in a space about the size of a water glass. These WIMPs pass through the Earth and our bodies every moment, but we cannot feel them at all because they interact very weakly.
Axion – A light but promising candidate
Axions are another candidate for dark matter. If they exist, they are particles that are billions of times lighter than electrons. They interact very weakly with light, and because their mass is so small, in order to satisfy the mass density of dark matter, there must be more than 10¹⁶ axions in a space the size of a water glass.
Axions are created in large quantities in a state of near-stasis during the process of quark-hadron transition, in which quarks that existed freely in the high temperatures of the early universe transform into protons and neutrons as the temperature decreases. This non-equilibrium creation process causes axions to move slowly despite their lightness, allowing them to gather into seeds of gravity during the formation of the universe. Thanks to this, they have the potential to aid rather than hinder galaxy formation.
Experiments to search for dark matter
Various experiments are actively underway to uncover the nature of dark matter. In the case of WIMPs, there are two methods of detection: direct detection and indirect detection. Direct detection involves using a detector to capture signals generated when WIMPs interact with atomic nuclei, which occurs very rarely.
Indirect detection, on the other hand, is a method of inferring the existence of WIMPs through the energy emitted when they annihilate in high-density regions (e.g., the center of a galaxy or the interior of the sun), i.e., light or high-energy particles (such as antiparticles). If more antiparticles than expected are detected in a specific energy spectrum, this could be a clue to the existence of WIMPs.
In addition, large particle accelerators are being used to conduct experiments to determine whether WIMPs can be created through proton collisions, which is also one of the key areas of dark matter research.
On the other hand, axions have the property of being able to transform into light under certain conditions in strong magnetic fields. Experiments to detect the light converted from axions are being conducted around the world, and the precision of this technology is rapidly improving.
Conclusion
Dark matter is still not fully understood, but it is a key element estimated to account for about 27% of the total mass of the universe. Its existence plays an important role in explaining various astrophysical phenomena, such as the rotation speed of galaxies, the formation of cosmic structures, and the interactions between galaxy clusters. Various particle candidates such as neutrinos, WIMPs, and axions have been proposed, and experiments to verify them are becoming increasingly precise.
As such, dark matter research not only deepens our understanding of the universe, but also drives human intellectual curiosity at the forefront of physics and astronomy. When its true nature is finally revealed, we will be able to see the universe with new eyes and expand our horizons.
