On a moonless night, the stars shine in the sky. It is hard for us to imagine that there are a lot of things called "dark matter" hidden in distant space, which actually account for the vast majority of the total mass of the universe, but they do not belong to any known substances that make up our visible celestial bodies.

Dark matter is invisible and intangible, but it has gravity and mass. So far, physicists still can't directly prove the existence of dark matter, but in the large-scale structure of the universe, the signs of dark matter are everywhere, and the galaxies are kept running stably without falling apart.

So, how did the dark matter hypothesis come about?

When the Swiss physicist Zwicky used the Doppler effect to measure the rotational speed of various places in spiral galaxies in 1933, he found that the orbital speed of celestial bodies from the center to the periphery did not decrease when they rotated around the center. According to the law of universal gravitation, there are dozens of times more invisible matter in this galaxy than visible stars and gas clouds. Our solar system is not like this. 99% of the mass of the solar system is concentrated on the sun, and the total number of unlit planets and comets is less than 1%. Planets close to the sun revolve around the sun rapidly. For example, the orbital speed of Mercury is about 48km/s, and the distant dwarf planet Pluto rotates slowly, only 4.7 km/s. ..

In the'1970s, Rubin of the United States measured the rotational speeds of more than 60 galaxies similar to the Milky Way, and reached the same conclusion as Zwicky. Later, a large number of new observation data also showed that there should be a lot of dark matter in galaxy clusters. Since then, the dark matter hypothesis has been widely recognized by academic circles.

What exactly is dark matter?

Scientists believe that if dark matter is composed of atoms like known ordinary matter, there should be 1000 billion black holes in the Milky Way. Either every star has a large number of planets, for example, the solar system should have 10000 Jupiter, but if only a few of these planets are heavier, they will fuse and glow, which is not dark matter.

At present, physicists generally believe that dark matter is not atoms, or even protons or neutrons, but "weakly interacting mass particles". In addition, there are also hypotheses that the main components of dark matter are composed of other types of particles, such as axions and inert neutrinos.

The observation results of modern astronomy through celestial motion, gravitational lens effect and microwave background radiation show that dark matter may exist in galaxies and the universe in large quantities, and its mass is far greater than the total amount of all visible substances. About 26.8% of the universe is dark matter, 4.9% is celestial bodies, interstellar gas and other ordinary substances, and the remaining 68.3% is dark energy that promotes the accelerated expansion of the universe.

How to detect the existence of dark matter?

At present, the detection of dark matter is mainly obtained through indirect evidence. As mentioned above, according to the gravitational theory of the motion of the spiral galaxy, the measured galaxy rotation curve shows that the only explanation is that there must be dark matter in the galaxy. X-rays produced by galaxy clusters can also be observed, and the mass distribution of dark matter in galaxies can be inferred from their temperatures.

The application of gravitational lens effect is a convenient way to indirectly detect dark matter. If the light of a star passes through pieces of dark matter on the way, the gravity of dark matter will bend the light again and again, so that some light that did not originally travel towards the earth will also reach the earth, making the star look much brighter.

According to the general theory of relativity, the gravitational lens effect can be confirmed as long as the light from behind the background passes through dark matter before reaching the earth. According to the bending degree of background light, the distribution of dark matter in galaxy clusters can be calculated, which is equivalent to obtaining evidence of the existence of dark matter.

Nowadays, scientists pay more attention to the experiment of directly detecting dark matter particles. For example, the "accelerator" is used to detect artificially generated dark matter particles in the particle collider. I hope that the secret of dark matter will be solved in the near future.

What is dark matter?

Dark matter is an invisible substance, which may theoretically exist in the universe. It may be the main component of cosmic matter, but it does not belong to any known matter that constitutes visible celestial bodies.

At present, a widely accepted theory holds that dark matter is composed of "weakly interacting particles with mass", and its mass and interaction intensity are near the electric weak scale. The observed residual abundance is obtained through the thermal decoupling process during the expansion of the universe.

There is no law of dark matter that requires matter to always be composed of charged particles, but for our eyes, matter that does not participate in electromagnetic interaction is invisible. The so-called dark matter has no electromagnetic charge at all, and no one can directly observe it with the naked eye or sensitive optical instruments.

The most striking sign of the existence of dark matter is precisely our human existence itself. Although invisible, dark matter is crucial to the evolution of our universe and the generation of stars, planets and even life. This is because dark matter carries five times the mass of conventional matter, and it does not directly interact with light.

These two properties are the key to determine that the structure of celestial bodies like galaxies can be formed in a short period of time in the life of typical galaxies as we know them, and the properties of dark matter are particularly critical to the formation of galaxies as big as our own Milky Way. If there is no dark matter, electromagnetic radiation will prevent galaxy structures from gathering for a long time.

Essentially, the possibility of galaxy structure formation is eliminated, and the universe will remain in a flat and uniform state. It is because of the existence of dark matter that the Milky Way, which is necessary for our life and the solar system, was formed in the time from the Big Bang to the present.

Extended data;

As far as dark matter is concerned, it can be traced back to the discovery of radioactive beta decay in the early 20th century. In order to explain this phenomenon, the Italian theorist Enrico Fermi assumed that there was a new force and a new force transfer particle in nature, which led to nuclear decay. This new force is similar to electromagnetic force.

This new force is now called weak nuclear force, and the hypothetical weak nuclear force transfer particles are W particles and Z particles. They are not dark matter in themselves, but their properties suggest the existence of dark matter. In fact, physicists believe that there may be dozens of particles waiting in line to be discovered-according to the so-called supersymmetry principle, each known particle has an unknown particle corresponding to it.

Among these hypothetical particles, there is a large class called weakly interacting massive particles, because these particles are named only by weak nuclear force interaction. However, whether these particles can really explain dark matter depends on their number, which is what really attracts the attention of particle physics.

Refer to Baidu Encyclopedia-Dark Matter

What is dark matter? How do scientists demonstrate its existence?

Dark matter is called dark matter because it does not interact with electromagnetic fields, that is, it does not absorb, reflect or emit electromagnetic radiation, so it is difficult to be detected. Modern astronomy relies on methods and theories such as gravitational lens, the formation of large-scale structure of the universe and microwave background radiation to detect dark matter.

Introduction to dark matter

The main evidence of dark matter comes from the calculation results, which show that many galaxies will fly away without a lot of invisible matter, or even will not form and will not operate as people have observed. Simply put, dark matter comes from calculation and theory and has not been directly observed. In the standard LambdaCDM universe model, the data detected by Planck satellite show that the universe consists of 4.9% conventional matter, 26.8% dark matter and 68.3% dark energy. So dark matter accounts for 85% of the total mass, and dark energy and dark matter account for 95% of the total mass and energy.

The Earth Surrounded by Dark Matter

The early history of dark matter

1884, Lord Kelvin estimated the mass of the Milky Way, and he determined that the mass of the universe is different from that of visible stars. Therefore, Lord Kelvin concluded that "many of our stars, perhaps most of them, may be dark matter". Kelvin, Poincare and other great men have speculated about the existence of dark matter, but these are speculations. It is the famous general relativity that provides a strong theoretical basis for dark matter. Later, Einstein and De Sitter co-wrote a paper about the existence of "invisible matter" in the universe. De Sitter is also known as "the theoretical pioneer of dark matter and dark energy".

1932, Dutch scientist Jan Oort presented evidence to infer the existence of dark matter. According to the motion of the stars in the Milky Way, he proposed that the Milky Way should have more mass, but the latter proof was said to be wrong.

In 1960s, the work of Vera Rubin, Kent Ford and Ken Freeman provided further strong evidence for dark matter. In addition, Rubin and Ford used a new spectrometer to measure the velocity curve at the edge of spiral galaxies more accurately. As we all know, the farther a planet is from a star, the slower its revolution speed. Based on this, rotating galaxies should also abide by this law.

But the results of their measurements were unexpected. The revolution speed of stars far from the center of galaxies is much faster than the theoretical value of Kepler's law. If you want to tie these extremely fast stars, you need more power. Where do these gravitational forces come from? Therefore, they speculate that there is a lot of mass that can pull the outer side of the galaxy so that it will not be separated from the galaxy due to excessive centrifugal force. This is the famous problem of galaxy rotation.

Dark matter classification

Historically, people have divided the possible dark matter into three categories: cold dark matter, warm dark matter and hot dark matter. This classification is not based on the real temperature of particles, but on their moving speed.

Cold dark matter: matter moving at classical speed.

Warm dark matter: particles move fast enough to produce relativistic effects.

Hot dark matter: Particle speed is close to the speed of light.

Detection of dark matter

Direct detection

The direct detection experiment of dark matter is generally set deep underground to eliminate the background noise of cosmic rays. At present, most experiments use cryogenic detectors or inert liquid detectors. Both detection techniques can identify the collision between dark matter and nucleon from the noise of collision between other particles and electrons.

Indirect detection

The indirect detection of dark matter is mainly to observe the signal produced by its annihilation. But before the background noise from other sources is fully understood, this kind of detection is not enough as decisive evidence of dark matter, so it can only be regarded as indirect detection.

abstract

Because dark matter has not been finalized, other hypotheses have emerged to explain the observed phenomena, so dark matter is also used to explain the observed phenomena. At present, the most commonly used method is to modify general relativity. General relativity has been well verified on the solar system scale, but its effectiveness on the galactic or cosmic scale has not been well proved. The mainstream view of most astrophysicists is that although the revision of general relativity can explain some observational evidence, there is enough data to conclude that there must be some form of dark matter in the universe. The direct detection experiment of dark matter is generally set deep underground to eliminate the background noise of cosmic rays.

What is dark matter?

With the further study of the Big Bang theory, dark matter appeared. So what exactly is dark matter? Let's be specific.

Recent astronomical observations and theoretical studies on the expansion of the universe show that the density of the universe may be composed of 70% dark energy, 5% luminous and non-luminous objects, 5% hot dark matter and 20% cold dark matter. That is to say, 90% of the universe is invisible dark matter, which may contain an invisible weak interaction heavy particle left over from the early universe-cold matter is the key to supporting the expansion theory of the universe.

It is precisely because dark energy and dark matter in the universe have not been discovered, so scientists have left us a series.

How much dark matter is there in the universe?

At present, scientists can't give specific figures, only some estimated figures: baryon matter accounts for about 2% of the total mass of the universe, that is to say, the total mass of all kinds of interstellar matter, stars and stars that can be observed in the universe only accounts for 2% of the total mass of the universe, and 98% of the matter has not been directly observed. In the dark matter of non-baryon matter in the universe, cold dark matter accounts for about 70%, and hot dark matter accounts for about 30%.

What is dark matter in the universe made of?

The Institute of High Energy Physics of the Chinese Academy of Sciences has always been in the international leading position in the search for dark matter. 1972, a strange phenomenon was observed by Yunnan Alpine Cosmic Ray Observatory of Institute of High Energy, that is, particles with energy greater than 300 billion electron volts from cosmic rays collided with one particle in graphite, resulting in three charged particles. The analysis shows that one of them is a negative meson, the other is a proton, and the other is a charged particle with energy greater than 43 billion electron volts and life longer than 0.046 nanosecond. Many scientists believe that if this substance can be confirmed, it will be a new particle beyond the standard model, and this new particle is probably the constituent particle of dark matter that has been searched for a long time.

In the 1980s, some scientists in the United States and the Soviet Union put forward the "axion" model of dark matter. According to this model, at the beginning of chaos, the universe is like an altar of baryon and axion mixed soup. Later, due to radiation energy, baryons slowly moved to the center of mass. As a result, the nucleus of ordinary luminous matter is surrounded by cold halo, forming a celestial body similar to a galaxy. Someone has simulated this model with a computer, and the final image of the evolution of the universe is very consistent with the universe observed by people today. However, due to the lack of theoretical support for this model, more experiments are needed to determine it.

What is dark matter?

With the further study of the Big Bang theory, dark matter appeared. So what exactly is dark matter? Let's be specific.

Recent astronomical observations and theoretical studies on the expansion of the universe show that the density of the universe may be composed of 70% dark energy, 5% luminous and non-luminous objects, 5% hot dark matter and 20% cold dark matter. That is to say, 90% of the universe is invisible dark matter, which may contain an invisible weak interaction heavy particle left over from the early universe-cold matter is the key to supporting the expansion theory of the universe.

It is precisely because dark energy and dark matter in the universe have not been discovered, so scientists have left us a series.

How much dark matter is there in the universe?

At present, scientists can't give specific figures, only some estimated figures: baryon matter accounts for about 2% of the total mass of the universe, that is to say, the total mass of all kinds of interstellar matter, stars and stars that can be observed in the universe only accounts for 2% of the total mass of the universe, and 98% of the matter has not been directly observed. In the dark matter of non-baryon matter in the universe, cold dark matter accounts for about 70%, and hot dark matter accounts for about 30%.

What is dark matter in the universe made of?

The Institute of High Energy Physics of the Chinese Academy of Sciences has always been in the international leading position in the search for dark matter. 1972, a strange phenomenon was observed by Yunnan Alpine Cosmic Ray Observatory of Institute of High Energy, that is, particles with energy greater than 300 billion electron volts from cosmic rays collided with one particle in graphite, resulting in three charged particles. The analysis shows that one of them is a negative meson, the other is a proton, and the other is a charged particle with energy greater than 43 billion electron volts and life longer than 0.046 nanosecond. Many scientists believe that if this substance can be confirmed, it will be a new particle beyond the standard model, and this new particle is probably the constituent particle of dark matter that has been searched for a long time.

In the 1980s, some scientists in the United States and the Soviet Union put forward the "axion" model of dark matter. According to this model, at the beginning of chaos, the universe is like an altar of baryon and axion mixed soup. Later, due to radiation energy, baryons slowly moved to the center of mass. As a result, the nucleus of ordinary luminous matter is surrounded by cold halo, forming a celestial body similar to a galaxy. Someone has simulated this model with a computer, and the final image of the evolution of the universe is very consistent with the universe observed by people today. However, due to the lack of theoretical support for this model, more experiments are needed to determine it.

What is dark matter?

With the further study of the Big Bang theory, dark matter appeared. So what exactly is dark matter? Let's be specific.

Recent astronomical observations and theoretical studies on the expansion of the universe show that the density of the universe may be composed of 70% dark energy, 5% luminous and non-luminous objects, 5% hot dark matter and 20% cold dark matter. That is to say, 90% of the universe is invisible dark matter, which may contain an invisible weak interaction heavy particle left over from the early universe-cold matter is the key to supporting the expansion theory of the universe.

It is precisely because dark energy and dark matter in the universe have not been discovered, so scientists have left us a series.

How much dark matter is there in the universe?

At present, scientists can't give specific figures, only some estimated figures: baryon matter accounts for about 2% of the total mass of the universe, that is to say, the total mass of all kinds of interstellar matter, stars and stars that can be observed in the universe only accounts for 2% of the total mass of the universe, and 98% of the matter has not been directly observed. In the dark matter of non-baryon matter in the universe, cold dark matter accounts for about 70%, and hot dark matter accounts for about 30%.

What is dark matter in the universe made of?

The Institute of High Energy Physics of the Chinese Academy of Sciences has always been in the international leading position in the search for dark matter. 1972, a strange phenomenon was observed by Yunnan Alpine Cosmic Ray Observatory of Institute of High Energy, that is, particles with energy greater than 300 billion electron volts from cosmic rays collided with one particle in graphite, resulting in three charged particles. The analysis shows that one of them is a negative meson, the other is a proton, and the other is a charged particle with energy greater than 43 billion electron volts and life longer than 0.046 nanosecond. Many scientists believe that if this substance can be confirmed, it will be a new particle beyond the standard model, and this new particle is probably the constituent particle of dark matter that has been searched for a long time.

In the 1980s, some scientists in the United States and the Soviet Union put forward the "axion" model of dark matter. According to this model, at the beginning of chaos, the universe is like an altar of baryon and axion mixed soup. Later, due to radiation energy, baryons slowly moved to the center of mass. As a result, the nucleus of ordinary luminous matter is surrounded by cold halo, forming a celestial body similar to a galaxy. Someone has simulated this model with a computer, and the final image of the evolution of the universe is very consistent with the universe observed by people today. However, due to the lack of theoretical support for this model, more experiments are needed to determine it.