Where does dark matter come from? Is there anything more mysterious in the universe than dark matter?

Scientists speculate that in this vast universe, there exists a mysterious substance that cannot be seen or touched, but it permeates every corner of the universe like a ghost and plays a vital role in the evolution of the universe. It is "dark matter."

In the 1930s, when astronomer Fritz Zwicky was studying the Coma Cluster, he discovered that the speed of galaxies in the cluster was much faster than they should be. This means that there should be more mass in the galaxy cluster to provide additional gravity to keep the galaxies in motion. Zwicky speculated that this extra mass might be an invisible substance, which he called "dark matter". However, the study of dark matter has been stagnant since then. It was not until the 1970s that American astronomer Vera Rubin discovered that the speed of the outer stars of spiral galaxies did not decrease as the distance from the center of the galaxy increased when studying the rotation curve of spiral galaxies, which was inconsistent with Newton's law of universal gravitation. Then there is a possibility that there may be a large amount of dark matter in the galaxy, which provides additional gravity, allowing the outer stars to move at such a high speed.

Dark matter is called "dark" matter because it does not interact with electromagnetic force and cannot emit, reflect or absorb light. Therefore, we have no way to observe it with conventional optical telescopes. This is why dark matter is still in the theoretical stage of hypothesis. When light passes through a massive celestial body, it will bend by the gravity of the celestial body. This phenomenon is similar to observing an object through a convex lens, hence the name "gravitational lens". If there is dark matter in the path of light, the gravity of the dark matter will also bend the light. By analyzing the degree of this distortion, scientists can calculate the mass and distribution of dark matter in galaxy clusters.

From this perspective, dark matter can be said to be very mysterious. So is there anything more mysterious than dark matter in the universe?

Scientists have calculated that observable matter in the universe accounts for only about 5% of the entire universe, while dark matter accounts for about 27% of the entire universe. Humans speculate that it has mass through its gravitational effect. But apart from mass, we have no way to observe and study this matter, but this is still good, at least we know that this 27% is still matter. Then, there is about 68% that is even more of a mystery to us. This 68% is called "dark energy" by scientists. You can imagine that it is a very large existence, but we don't know what it is. Since we don't know what it is, why do we call it dark energy?

Scientists have discovered that the universe is constantly expanding, and the speed of expansion is accelerating. This phenomenon is contrary to existing physics theories, because according to theory, gravity should gradually slow down the expansion of the universe, and even eventually begin to shrink. But the actual situation is that the universe is expanding at an accelerated rate, so astronomers believe that there may be a mysterious force that is pushing everything in the universe outward, causing the universe to expand at an increasingly faster rate. We call this mysterious force "dark energy."

Through computer simulations, scientists can trace back from the current state of the universe to the beginning of the Big Bang to see how much dark matter and dark energy are needed to turn the universe into what we see now. If the appropriate proportion of dark matter is not set in the simulation, then the galaxy shape we see now cannot be obtained. If dark matter and dark energy are mixed together in the simulation, that is, if dark energy does not exist and dark matter accounts for 95%, there is no way to get the same answer in the simulation. Therefore, the proportion of conventional matter, dark matter and dark energy in the universe is approximately 5%, 27% and 68%.

So how should we detect dark matter and dark energy?

In fact, the most direct one is the "gravitational lens" effect we mentioned earlier, which can outline the distribution of dark matter by observing the bending of light when passing through certain areas. In addition, we can also detect through cosmic rays. For example, my country's "Wukong" dark matter particle detection satellite can measure high-energy cosmic rays. If it can find cosmic ray sources other than supernova explosions, it may also indirectly detect dark matter. In addition, underground laboratory detection is one of the most direct dark matter detection methods at present. Because the interaction between dark matter and ordinary matter is very weak, it needs to be detected in an extremely quiet environment. Underground laboratories can effectively shield interference from cosmic rays and the earth's surface, providing an ideal environment for dark matter detection. At present, there are many underground laboratories in the world that are conducting dark matter detection experiments. For example, our country's Jinping Underground Laboratory, located 2,400 meters underground in Jinping Mountain, Liangshan Prefecture, Sichuan, is one of the deepest underground laboratories in the world. Here, scientists are conducting a series of dark matter detection experiments, hoping to find direct evidence of the existence of dark matter.

As for the detection of dark energy, the most mainstream method at present is to observe the cosmic microwave background radiation, which is the weak electromagnetic radiation left over from the Big Bang. Its distribution and characteristics contain information about the early state of the universe. Through the precise measurement of the cosmic microwave background radiation, we can understand the distribution of matter and energy in the universe, and thus provide clues for the study of dark energy.

Although scientists have been studying dark matter and dark energy for decades, we still know very little about the details of their "real" nature, such as what kind of particles are dark matter? Is dark energy the cosmological constant? But at least we know that dark matter and dark energy are one of the most important components of the universe, and their existence directly affects the structure and evolution of the universe. Perhaps, the day when we truly unravel the mystery of dark matter and dark energy, we can have a deeper understanding of the nature of the universe. Perhaps, this involves the ultimate question of our human philosophy, who are we? Where do we come from? Where are we going?

Author: Stars Become Light

Reviewer: Zhou Binghong, researcher at the National Space Science Center of the Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.