It seems we have been fooled, the universe is hiding it! What is dark matter?

We searched for it, but it was nowhere to be found...

Most matter in space is invisible to the naked eye, but its effects on large celestial structures can be detected.

This composite image shows the distribution of dark matter, galaxies, and hot gas in the center of the merged galaxy cluster Abell 520, which was formed by a violent collision between massive galaxy clusters. In the center of the image, where the blue and green light mix, dark matter clumps around most of the hot gas, and few galaxies, are visible. [1-2]

What is dark matter?

Astronomers estimate that about 85% of the matter in the universe is dark matter, and the remaining 15% is ordinary matter. If dark energy, which accelerates the expansion of the universe, is taken into account, the Nuclear Research Organization estimates that dark matter accounts for about 27% of the total mass-energy of the universe.

(According to many scientific research studies, it is generally believed that the universe is composed of matter and energy, about 70% of the universe is dark energy, and the rest is ordinary matter and dark matter.)

Astronomers have different ways of measuring the total mass of matter in the universe and the ratio between the total mass of matter in the universe and the mass of ordinary matter (also known as baryonic matter). The simplest way is to compare two different measurement results.

The first is to measure the total amount of light released by large astronomical structures, such as galaxies, from which the mass of the structure can be inferred.

The second method is to infer the mass of large celestial bodies based on the gravity required to maintain the stability of their structures. Astronomers use them to measure the mass of galaxies or galaxy clusters throughout the universe. After multiple measurements and comparisons, they have found that the gravity generated by the mass of ordinary luminous matter is not enough to bind these luminous substances. This undoubtedly shows that there is non-luminous matter, which we call dark matter.

(According to the characteristics of gravity, in simple terms, there is mutual attraction between objects, and the magnitude of gravity is proportional to the mass of the object, that is, the greater the mass, the greater the gravity; the speed of the celestial body is inversely proportional to the square root of the orbital radius, that is, the larger the radius, the smaller the speed. However, the actual observation comparison results show that the speed of the outer celestial bodies has not decreased but is larger than expected, and the gravity generated by the mass of the galaxy is not enough to bind them, which theoretically means that there are still other invisible substances exerting gravity.)

The ratio of dark matter to ordinary matter varies from galaxy to galaxy. Some galaxies contain almost no dark matter, while others contain almost no ordinary matter. But the average results of many measurements show that about 85% of the matter in the universe neither emits light nor interacts with light.

I've searched everywhere for it, but still can't find it.

Despite this, astronomers still have many ways to confirm the existence of dark matter. For example, massive celestial structures can distort the surrounding space-time, such as galaxy clusters. When light passes through them, it will be affected and deflected, and focused at the other end. This phenomenon is called the gravitational lens effect. The mass of the celestial body inferred by this effect is compared with the mass of the luminous celestial body we directly see. It is found that the total mass of matter that actually produces gravity is much greater than the amount of luminous matter. The result once again shows that the "missing" mass must be lurking somewhere.

Cosmologist Joel Primack once mentioned in a speech that dark matter can gather ordinary matter to make galaxies grow continuously. With the help of computer simulation technology, the evolution of large celestial structures can be simulated and observed. Billions of years ago, our universe was much smaller than it is today. During this period, stars and galaxies continued to evolve. If it relied solely on the role of ordinary visible matter, it would not be enough to form the galaxies we see today.

Cosmologists also tried to calculate from the early days of the universe. In the first dozen minutes, the universe produced the first batch of protons and neutrons. Based on nuclear physics knowledge, the production of hydrogen and helium in that period can be estimated. These calculation results can accurately estimate the ratio of hydrogen and helium in the current universe. Astrophysicist Ned Wright said that they can also predict the upper limit of the amount of baryonic matter in the universe, and the inferred data is consistent with the actual observations of galaxies and galaxy clusters today.

Looking back, does the dark matter that I’ve been thinking about really exist?

Or, under Newton's laws and Einstein's general theory of relativity, the conjecture about the existence of dark matter may be wrong. Perhaps the theory of gravity is still not perfect enough, and astronomers have tried not to introduce the concept of dark matter, but only to make minor adjustments to the theory of gravity. Although the adjusted theory of gravity can explain the observations based on the conjecture of the existence of dark matter in some cases, such as the movement of stars in galaxies, the adjusted theory of gravity still cannot well explain all the observed phenomena in the universe. [3]

All observational data tend to suggest that dark matter is some unknown particle that interacts neither with light nor with ordinary matter, and its existence can only be inferred indirectly through gravity. Astronomers believe that trillions of dark matter particles are constantly passing through us, but we cannot perceive them.

The journey of exploration is full of unknowns. We hope that scientists will one day be able to uncover the mystery of the composition of the universe.

Related knowledge

The universe is a unified entity consisting of time, space, energy and matter (including planets, stars and other forms of matter). The generally accepted view on how the universe was born and developed is the Big Bang theory. According to the theory, time and space began with a big bang about 13.8 billion years ago, and the universe has been expanding since then. Therefore, the boundary of the entire universe is unknown, but the diameter of the observable universe can be detected, which is almost 93 billion light years long.

A galaxy is a celestial system formed by the gravitational pull of stars, stellar remnants, interstellar gas, dust and dark matter. Galaxy comes from the Greek word "galaxias (γαλαξίας)", which means "milk". The Milky Way galaxy, which includes the solar system, also gets its name from this.

When the ancient Greeks looked up at the long river of silver light in the night sky, they imagined that a god must have spilled milk in a certain direction, and the milk on the horizon became the Milky Way. Adapted from A Brief History of the Universe (Illustrated Edition)

Galaxies range in size from dwarf galaxies with tens of millions of stars to supergiant galaxies with hundreds of trillions of stars. On average, each galaxy has more than 100 million stars, each orbiting around its galactic mass center. In a typical galaxy, dark matter accounts for the vast majority of its material mass, while stars and nebulae, which are visible to the naked eye, account for a very small proportion of the mass. In addition, most galaxies may have supermassive black holes at their cores.

BY: Paul Sutter

FY: YMD

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