What is Dark Matter?

Dark matter is an invisible substance that may exist in the universe. It may be the main component of cosmic matter, but it does not belong to any of the currently known substances that make up visible celestial bodies. The suspected violation of Newton's gravitation found in a large number of astronomical observations can be well explained on the assumption that dark matter exists. Modern astronomy through the motion of celestial bodies, the phenomenon of Newton's universal gravitation, the effect of gravitational lensing, the formation of large-scale structures of the universe, microwave background radiation, and other observations indicate that dark matter may exist in galaxies, star clusters, and the universe in large quantities, and its mass is far greater than all The total mass of the celestial body can be seen. Combined with the anisotropy observation of microwave background radiation in the universe and the standard cosmological model (CDM model), it can be determined that dark matter in the universe accounts for 85% of the total mass of the material and 26.8% of the total mass energy of the universe.

The earliest proposed "dark matter" may be astronomer Jacobus Kapteyn, who proposed in 1922 that the invisible matter around the stars could be indirectly inferred from the motion of the star system ^[1]

Although dark matter has not been detected directly, there is already a lot of evidence that it exists in the universe in large numbers, such as:

The existence of dark matter has been widely recognized, but currently little is known about the properties of dark matter. The currently known properties of dark matter include only a few aspects:

(1) Dark matter participates in gravitational interactions, so it should have mass, but a single dark matter

Weakly interacting mass particles (WIMP) is one of the most widely discussed candidates for dark matter. It refers to a kind of stable particles with mass and interaction strength near the electric weak scale. It is currently known through thermal decoupling Remaining abundance. WIMP should be basically electrically neutral and color neutral, so it does not directly participate in electromagnetic and strong interactions. Neutrinos also do not participate in strong and electromagnetic interactions, but because they move at close to the speed of light in the universe, they belong to "thermal dark matter" and are not enough as the main component of dark matter. At present, in the standard model of particle physics known to man, there are no particles that meet these properties at the same time, which means that WIMP must be a new physical particle that exceeds the standard model. WIMPs that have been predicted by theory include: the lightest supersymmetric partner particle in a supersymmetric model ^[14]

Even if the dark matter particles have only a weak interaction with conventional matter, the dark matter particles may be detected by sophisticated experimental equipment. The detection methods currently used by scientists can be divided into three categories: one is to detect dark matter particles that directly interact with the material in the detector, which is called "direct detection"; the other is to look for signals that the dark matter itself decays or annihilates in the universe to produce common matter , Called "indirect detection", and the third is to search for artificially produced dark matter particles in the particle collider, called "accelerator detection".

(1) Direct detection. If dark matter is made up of microscopic particles, there should be a large number of dark matter particles passing through the earth at all times. If one of the particles hits the nucleus in the detector's material, the detector can detect the change in the energy of the nucleus and understand the properties of the dark matter by analyzing the nature of the impact. However, for weakly interacting mass particles (WIMPs), because their interaction with ordinary matter is extremely weak, the probability of being captured by the detector is very weak. In order to maximize the shielding of other types of cosmic rays, direct dark matter detection experiments are often performed deep underground. At present, there are dozens of dark matter underground exploration experiments in the world. There is no conclusive evidence that direct detection experiments have found dark matter particles. The results of these experiments strongly limit the mass and interaction strength of dark matter particles.

(2) Indirect detection. Since there are a large number of dark matter particles in the Milky Way, it should be possible to detect the conventional elementary particles produced by their annihilation or decay. Indirect detection is to find such annihilation or decay signals in astronomical observations, including high-energy gamma rays in cosmic rays Horse rays, positrons and positrons, positive and negative protons, neutrons, neutrinos, and various cosmic ray nuclei. Experiments using indirect detection methods can be the direct collection of cosmic ray particles using space probes carried by satellites or space stations, or the observation of showers or Cherenkov light effects produced by high-energy cosmic ray particles entering the Earth's atmosphere on the ground. By analyzing the number and energy spectrum of various particles in the cosmic ray, information on the decay or annihilation of dark matter in the universe can be extracted. The difficulty of indirect detection of dark matter is that there are many high-energy ray sources in the universe that are not produced by dark matter, and that cosmic rays undergo a complex propagation process from the time they are generated until they reach the earth. The current understanding of the generation and propagation of cosmic rays is incomplete, which poses a challenge for finding dark matter signals in cosmic rays. There are currently many dark matter space exploration experiments in progress around the world.

(3) Collider detection. Another way to find dark matter is to produce dark matter particles in the laboratory. In high-energy particle collision experiments, particles that have not yet been discovered, including dark matter particles, may be generated. If the collision produces dark matter particles, it is difficult to be directly detected by the detector, which will cause the total energy and momentum of the collision product particles detected by the detector to be lost. This is a feature that produces invisible particles. Combined with direct or indirect detection methods, it can help determine whether the particles generated in the collider are dark matter particles.