If the universe is compared to a fish tank, dark matter is the water inside, so what is the nature of dark matter?

In a fish tank with clear water, you can see everything, fish, landscaping, aquatic plants, etc., and the only thing you can't see is water.

This is a very interesting thing. Water is everywhere in the fish tank, but we just can't see it. If we compare the universe to this fish tank, then dark matter is the water inside. What is dark matter? Our universe is made up of matter, but the "matter" here does not only refer to the matter we can observe, but also may include another type of invisible matter, that is, dark matter. In terms of total amount, dark matter is likely to be much more than matter.

The name "dark matter" can be said to be very appropriate.

Matter is visible because it can reflect light, while dark matter is characterized by not emitting light itself, nor absorbing or reflecting light, so it is completely invisible. So the question is, since dark matter is completely invisible, how can we confirm its existence? Confirming the existence of dark matter mainly depends on the gravitational force between matter. Any matter with mass has mutual gravitational force, and dark matter is no exception. Therefore, the widespread existence of dark matter will inevitably affect the behavior of celestial bodies and even galaxies.

The movements of many celestial bodies and galaxies observed by scientists are different from the estimated results, and the reason for this difference is probably the existence of some invisible matter that produces gravitational influence.

Of course, this is just speculation, so how can we confirm the existence of dark matter? To confirm the existence of dark matter, we must figure out what dark matter is, that is, to uncover the essence of dark matter. Scientists generally believe that dark matter is some unknown elementary particle, and there is no dispute on this point. As for what this elementary particle is, we still don’t know, because all the detections of dark matter particles have ended in failure so far.

There are currently two main speculations about the nature of dark matter. One is that dark matter is a weakly interacting massive particle, and the other is that dark matter particles are a hypothetical particle called "axion."

There is a huge difference between these two conjectures. The former believes that dark matter particles are massive elementary particles that can only interact through gravity, and their mass is at least 10 times that of protons. The latter believes that axions are very light elementary particles, even much lighter than the lightest known elementary particles, and they propagate in space in the form of waves.

For a long time, people were unable to decide which of the two hypotheses was better, but recent studies have tipped the balance in favor of the axion.

The essence of gravity is a geometric effect that causes the surrounding space-time to bend, which is very obvious around massive matter, such as galaxies. There is obvious curvature of space-time around galaxies, which can be confirmed by observing the light around distant galaxies. Astronomers have discovered through continuous observation that the curvature of space-time around distant galaxies is wrinkled, which just fits the axion conjecture. Because if dark matter is a weakly interacting massive particle, then the curvature of space-time around galaxies should be smooth.

Alfred Amruth and his research team at the University of Hong Kong also found similar astrophysical evidence.

Amruth and his team focused on an elliptical galaxy numbered HS 0810+2554. The light of this galaxy passes through another galaxy before reaching the earth, so its image will be distorted due to the gravitational lens effect. By modeling and calculating the degree of distortion, Amruth found that if dark matter is composed of axions, then the light of this galaxy will propagate in a chaotic path, and the actual observation results coincide with this conclusion, which further confirms that dark matter is composed of axions. Of course, the current research is not enough to draw a definite conclusion on the nature of dark matter. There is still a long way to go to peek into the nature of dark matter and confirm its existence.