What is dark matter in the universe?

What is dark matter? Dark matter (including dark energy) is considered as the most challenging subject in the study of the universe, which represents more than 90% of the matter content in the universe, while the matter we can see accounts for less than 10% (about 5%) of the total mass of the universe. Dark matter cannot be directly observed, but it can interfere with the light waves or gravity emitted by stars and can clearly feel its existence. Scientists have put forward many hypotheses about the characteristics of dark matter, but they have not been fully proved so far.

Decades ago, when dark matter was first proposed, it was only a product of theory, but now we know that dark matter has become an important part of the universe. The total mass of dark matter is 6.3 times that of ordinary matter, accounting for 1/4 of the cosmic energy density. More importantly, dark matter dominates the formation of the cosmic structure. The nature of dark matter is still a mystery, but if it is assumed to be a weakly interacting subatomic particle, then the large-scale structure of the universe formed from it is consistent with the observation. However, the recent analysis of the structure of galaxies and sub-galaxies shows that there is a difference between this hypothesis and the observed results, which also provides a useful place for many possible dark matter theories. These potential dark matter models can be distinguished by studying the density, distribution, evolution and environment of small-scale structures, which brings new dawn to the study of dark matter properties.

About 65 years ago, evidence of the existence of dark matter was first discovered. At that time, Fritz Zwicky found that the galaxies in the large galaxy cluster had extremely high moving speed, and unless the mass of the galaxy cluster exceeded 100 times the value calculated according to the number of stars in it, the galaxy cluster could not bind these galaxies at all. After decades of observation and analysis, this has been confirmed. Although we still know nothing about the nature of dark matter, by the 1980s, dark matter, which accounts for about 20% of the energy density of the universe, was widely accepted.

Dark energy

In cosmology, dark energy is a conjecture of some people, which refers to an energy that is full of space and has a strong negative pressure. According to the theory of relativity, this negative pressure is similar to the long-distance antigravity. Today, this conjecture is the most popular solution to explain the accelerated expansion of the universe and the loss of matter in the universe.

There are two main models of dark energy: cosmological constant (that is, constant energy density that fills space evenly) and essence (that is, dynamic field in which energy density changes with time and space). Distinguishing the two may require a high-precision measurement of the expansion of the universe and a deeper understanding of the change of the expansion speed with time. Because the expansion speed of the universe is described by the equation of state in cosmology, measuring the equation of state of dark matter is one of the most important problems in observational cosmology.

Dark energy is an invisible energy that can promote the movement of the universe. The motion of all stars and planets in the universe is driven by dark energy. The reason why dark energy is so powerful is that it accounts for about 73% of the structure of the universe and occupies an absolute dominant position. Dark energy is a milestone in cosmological research in recent years. There are two main evidences to support dark energy. First of all, a large number of observations of distant supernovae show that the universe is accelerating its expansion. According to Einstein's gravitational field equation, the phenomenon of accelerated expansion infers that there is negative pressure "dark energy" in the universe. Another evidence comes from the study of microwave background radiation in recent years, which accurately measures the total density of matter in the universe. We know that all ordinary matter and dark matter add up to only about 1/3, so we still need about 2/3. The shortage of this substance is called dark energy, which is characterized by negative pressure, almost uniform distribution in space or no aggregation at all. According to the latest WMAP data, dark energy accounts for 73% of the total matter in the universe. It is worth noting that for ordinary energy (radiation), baryon and cold dark matter, the pressure is non-negative, so there must be an unknown negative pressure substance that dominates the universe today.

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