Weakly interacting mass particles (WIMPs) are one of the most widely discussed candidates for dark matter, and it refers to a stable particle with mass and interaction intensity around the electroweak scale, and the currently known residual abundance is obtained through a thermal decoupling mechanism. WIMP should be essentially electrically and color-neutral, and therefore not directly involved in electromagnetic and strong interactions.

Neutrinos are also not involved in strong and electromagnetic interactions, but because they move at nearly the speed of light in the universe, they belong to "hot dark matter" and are not sufficient as the main component of dark matter.

There is no particle in the Standard Model of particle physics that satisfies both of these properties, which means that WIMP must be a new physical particle beyond the Standard Model.

The WIMP that has been theoretically predicted includes: the lightest supersymmetric companion particle in the supersymmetric model, such as the superneutron (

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The minimum Kaluza-Klei in the theory of extra dimensions

Excited particles; T-odd particles in the Little Higgs model.

Another dark matter candidate is axon (axio

), a very light neutral particle, which is associated with the joint symmetry breaking of charge conjugation-cosmic inversion in strong interactions.

The axions interact with each other by very small forces, so that they cannot be in thermal equilibrium with the background radiation, so the residual abundance is not obtained by thermal decoupling, but it can become cold dark matter by breaking the vacuum state.

Although many astronomical observations have been made about dark matter, its composition is still not fully understood. Early dark matter theories focused on hidden conventional material stars, such as black holes, neutron stars, aging white dwarfs, brown dwarfs, etc.

These stars are generally classified as MAssive Compact Halo Objects (MACHOs), but astronomical observations over the years have not been able to find sufficient amounts of MACHOs.

It is thought that hard-to-detect baryonic matter (e.g., MACHOs and some gases) do contribute to some of the dark matter effects, but evidence suggests that such substances account for only a small fraction of them.

The rest is called "non-baryon dark matter". In addition, observational data such as galaxy rotation curves, gravitational lensing, cosmological structure formation, the proportion of baryons in galaxy clusters, and the abundance of galaxy clusters (combined with independent evidence of baryon density) also point to 85%-90% of the mass in the universe not participating in electromagnetic interactions.

This type of "non-baryonic dark matter" is generally assumed to be composed of one or more elementary particles that are different from conventional matter (electrons, protons, neutrons, neutrinos, etc.).

Since there is no direct detection evidence of the existence of dark matter, there are also theories that attempt to explain the existing astronomical observations without introducing dark matter. A typical class of theories is the Modified Newto's theory of gravitation

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amics, MOND), which argues that Newton's or Einstein's theories of gravity are incomplete, and that gravity behaves differently at different scales. However, the evidence for the existence of dark matter comes from many unrelated observations, and it is very challenging to explain all of them simultaneously through the theory of gravity without introducing dark matter.

In particular, the apparent separation of visible matter from the center of mass of the colliding star clusters observed in the case of the "Bullet Cluster" is observational evidence that the theory of dark matter exists rather than gravity needs to be revised.

Even if there is only a slight interaction between dark matter particles and conventional matter, dark matter particles can be detected by sophisticated experimental instruments. At present, the detection methods used by scientists can be divided into three categories: one is to detect the direct interaction of dark matter particles with the matter in the detector, which is called "direct detection"; the second is to find the signal that the dark matter itself decays or annihilates in the universe to produce ordinary matter, which is called "indirect detection", and the third is to explore the artificially generated dark matter particles in the particle collider, which is called "accelerator detection".

1. Direct detection. If dark matter is made up of microscopic particles, then 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 of the detector's matter, the detector will be able to detect changes in the energy of the nucleus and learn about the properties of the dark matter by analyzing the nature of the impact. However, for weakly interacting mass particles (WIMPs), the probability of being caught by the detector is also very weak due to the extremely weak interaction between them and ordinary matter.

In order to shield against other kinds of cosmic rays as much as possible, experiments on direct detection of dark matter are often carried out deep underground. Currently, there are dozens of underground experiments on dark matter in progress around the world.

There is no conclusive evidence of the existence of dark matter particles in direct detection tests. The results of these experiments strongly limit the mass and interaction intensity 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, and indirect detection is to look for such annihilation or decay signals in astronomical observations, including high-energy gamma rays, positrons and negative electrons, positive and negative protons, neutrons, neutrinos, and various cosmic ray nucleons in cosmic rays.

Experiments using indirect detection methods can be the direct collection of cosmic ray particles by space probes on satellites or space stations, or the observation of clusters or Cherenkov light effects from the ground when high-energy cosmic ray particles enter the Earth's atmosphere.

By analyzing the number and energy spectrum of various particles in the cosmic rays, it is possible to extract information about the decay or annihilation of dark matter in the universe. The difficulty of indirect detection of dark matter lies in the fact that there are many high-energy ray sources in the universe that are not produced by dark matter, and cosmic rays undergo a complex propagation process from generation to arrival near the Earth.

At present, the understanding of the generation and propagation process of cosmic rays is not comprehensive, which brings challenges to the search for dark matter signals in cosmic rays. At present, there are a number of dark matter space exploration experiments in the world.

3. Collider detection. Another way to find dark matter is to create dark matter particles in the lab. In high-energy particle collision experiments, undiscovered particles, including dark matter particles, may be produced. If a collision produces a dark matter particle, it will be difficult to be directly detected by the detector, resulting in a loss of total energy and momentum of the collision product particles detected by the detector. This is a characteristic that produces invisible particles. Combined with direct or indirect detection, it can help determine whether the particles produced in the collider are dark matter particles.

The Milky Way is the star system where the solar system is located, including 1500~400 billion stars and a large number of star clusters and nebulae, as well as various types of interstellar gas and interstellar dust, black holes, and its total visible mass is 210 billion times the mass of the sun.

When this information was roughly read by Hua Feng, new information flows followed. Hua Feng noticed that this time it was about the Milky Way.

"Great Sage, what does this mean?" Hua Feng was very puzzled by the information instilled by Sun Wukong for no reason.

"Don't worry about it for now, just keep this information in your head, and you'll know why I'm doing it later. Now this era is different from mine, and it took me a hundred years to adapt to the world again. Now that the world is about to be threatened like never before, I have to do something. When Sun Wukong said this, his tone had become a little excited.

When Hua Feng heard this, he didn't ask any more questions, and continued to digest this massive flow of information.

Most of the stars in the Milky Way are concentrated in an oblate sphere of space, which is shaped like a discus. The protruding part of the oblate sphere in the middle is called the "nuclear sphere" and has a radius of about 7,000 light-years. The middle part of the nuclear ball is called the "silver core", and the surrounding area is called the "silver plate". Outside the galactic disk there is a larger, spherical region where there are fewer stars and less dense, known as the "galactic halo", with a diameter of 70,000 light-years.

In the past, the Milky Way was thought to be a spiral galaxy like the Andromeda Galaxy, but the latest research suggests that the Milky Way should be a barred spiral galaxy.

90% of the Milky Way is made up of stars. There are many types of stars, and according to their physical properties, chemical composition, spatial distribution, and motion characteristics, stars can be divided into five star families. The youngest extreme family I. stars are mainly distributed in the spiral arms of the galactic disk, and the oldest extreme family II stars are mainly distributed in the galactic halo. Stars often clump together. In addition to a large number of binary stars, more than a thousand star clusters have been discovered in the Milky Way. There is also gas and dust in the Milky Way, which accounts for about 10% of the total mass of the Milky Way, and the distribution of gas and dust is uneven, some are clustered into nebulae, and some are scattered in interstellar space.

Since the 60s of the 20th century, a large number of interstellar molecules, such as carbon monoxide, water, etc., have been discovered.

Molecular clouds are the main site of star formation. The core of the Milky Way, the galactic core, or the galactic core, is a very special place. It emits intense radio, infrared, X-ray and γ-ray radiation, the nature of which is unknown, and there may be a giant black hole there, estimated to have a mass of up to 4 million times the mass of the Sun.

In 1971, British astronomers Lyndon Bell and Martin Ness analyzed infrared observations and other properties in the central region of the Milky Way, pointed out that the energy source of the center of the Milky Way should be a black hole, and predicted that if their hypothesis was correct, a small source of radio radiation should be observable at the center of the Milky Way, and the properties of this radiation should be the same as that observed in terrestrial synchrotrons. Three years later, such a radiation source was discovered, and this was Sagittarius A.