Dark matter is a scientific concept, it is a mysterious, not directly observed matter in the universe. Scientists have inferred the existence of dark matter mainly because of its gravitational effect on the universe. The existence of dark matter can explain many astronomical observations, such as the rotation speed of galaxies and the structure of galaxy clusters.
First, we need to understand what dark matter is. Dark matter is a hypothetical substance that is different from the ordinary substances that we are exposed to in our daily life and can be directly detected in scientific experiments, such as electrons, protons, neutrons, etc. Dark matter does not emit any electromagnetic radiation, including light, so we cannot see dark matter directly with equipment such as optical telescopes.
So, how do we know dark matter exists? This is thanks to its gravitational effect. While we can't see dark matter directly, we can infer its presence from its gravitational effect on other matter. For example, by observing the rotation speed of galaxies, we found that the rotation speed of the outer stars of galaxies is much higher than the speed calculated according to the distribution of visible matter. It's like you're throwing a rock with a rope. If the rope is long enough, you need to use more force to keep the rock spinning. In the same way, the rotation of galaxies requires an invisible "rope" to provide enough gravitational force, which is dark matter.
In addition, dark matter also affects the large-scale cosmic structure. Our universe is made up of countless galaxies and galaxy clusters, which are not evenly distributed, but form a unique network structure. The formation of this structure requires a large amount of matter to provide gravity, and most of this matter is dark matter.
Despite some understanding of dark matter, its nature remains a mystery. Some scientists believe that dark matter is composed of one or several unknown particles called dark matter particles. Scientists are conducting various experiments to try to detect dark matter particles directly, but so far without success. Other scientists believe that dark matter may be a manifestation of our incomplete understanding of the theory of gravity, that is, we may need a new theory of gravity to explain the phenomenon of dark matter.