Since dark matter is present throughout the universe, why can't humans perceive it, yet know it exists?

To clarify this issue, we need to first clarify three basic questions, namely: 1. How do humans perceive the world? 2. How does dark matter exist? 3. Have humans sensed the existence of dark matter?

Once these three small issues are clarified, the big problem will be solved. Now let me talk about my views, that is, to share the known scientific common sense with you in a popular way through my understanding.

Let's first talk about how humans perceive the world. There are four basic forces in the world, namely strong interaction, weak interaction, electromagnetic interaction, and gravitational interaction, referred to as strong force, weak force, electromagnetic force, and gravitation. These four forces dominate all movements in the world, that is, they are all-encompassing and cannot be separated from these four basic forces, including all activities of you, me, and him in this world.

The main way humans perceive the world is through electromagnetic force, which is the most common force in our daily lives. Do not narrowly understand electromagnetic force as electric current or radio frequency action. All chemical and physical phenomena that we understand or do not understand are manifestations of electromagnetic force.

Why do we say this? Because the strong and weak forces only act on the atomic and subatomic levels, and do not exist beyond the atomic level; gravity is everywhere, and wherever there is mass, there will be gravity, but gravity is the weakest of the four forces. Except for us being attracted to the surface of the earth by gravity, it is difficult for people to feel it at ordinary times.

In this way, we humans directly perceive all aspects of the world, including eating, drinking, defecating, urinating, sitting still, or meeting people, talking, and shaking hands, all through the interaction of electromagnetic forces.

Theoretically, electromagnetic force is the interaction between charged particles or particles with magnetic moments transmitted through electromagnetic fields, also known as Lorentz force, and the medium of transmission is photons. We know that all matter in this world is composed of atoms, and atoms are composed of nuclei and electrons, which carry electric charges themselves. It is only because the positive and negative charges of nuclei and electrons are equal that atoms appear neutral.

When atoms collide and come into contact, the electrons on the surfaces of the atoms in contact interact with each other, generating electromagnetic stimulation, which gives us sensations. Human vision, hearing, smell, touch, and thinking are all the result of electromagnetic force.

Vision relies on photon transmission; hearing is a mechanical wave, which is transmitted by the vibration of molecules and atoms in the transmission medium; smell is the result of the molecules and atoms of a substance volatilizing in the air contacting the human olfactory organs and emitting electrical stimulation; touch is a manifestation of friction, which is the result of the conduction of electromagnetic force generated by the contact between molecules on the surface of the body and molecules of other objects; thinking is the result of the movement of brain waves in structures such as nerve cells and synapses.

All the radio waves, light, heat and other things we are exposed to are the result of electromagnetic force. The electromagnetic force is second only to the strong force, and theoretically it can propagate infinitely far, so we can see galaxies over 13 billion light years away through telescopes.

Theoretically, any substance that has electromagnetic force and can interact with photons can be discovered or seen by humans. For example, if an object reflects, refracts, or diffracts under light, it can be captured by the human eye or electromagnetic instruments; or if it can interact with the atoms and molecules of matter, it can also be observed and felt.

But dark matter has no electromagnetic force at all. First of all, dark matter does not interact with photons, does not emit any reflection, refraction, scattering, or diffraction under light, is completely transparent to light, and cannot be observed by the human eye. This at least shows that it is not composed of charged elementary particles such as atoms or subatomic particles, because as long as these particles are charged, they will be bound by electromagnetic force and will interact with various charged particles, including molecules, atoms, protons, electrons, and photons.

Based on this, dark matter is not only invisible, but also cannot be sensed by human touch, hearing, and smell. Therefore, in theory, we are surrounded by dark matter, but we cannot perceive it.

After years of observation and research, the scientific community has made a relatively accurate calculation of the visible and invisible matter in the universe, that is, in the entire universe, visible matter only accounts for 4.9% of the total mass-energy, and invisible mass-energy accounts for 95.1%. Why is it the total mass-energy? Because in the invisible mass-energy, not all is dark matter. Dark matter only accounts for 26.8%, and dark energy accounts for 68.3%.

Dark matter and dark energy play opposing roles in the universe. Dark matter plays a gravitational role of pulling and gathering; dark energy plays an expansion role of repelling and expanding. One pulls and gathers, the other repels and expands. The two forces have been in a game. At present, dark energy has won. It is still unpredictable who will win in the end.

The magnitude of universal gravitation is proportional to mass. Dark matter accounts for 84.5% of visible and invisible matter, and therefore dominates the gravitational force of galaxies; while dark energy dominates the expansion of the universe. Its mass-energy is more than twice that of dark matter. Therefore, the gravity of dark matter cannot shrink the universe. Instead, the entire universe is dominated by dark matter and shows an expansion trend.

Neither dark matter nor dark energy has electromagnetic force, so neither can be directly observed.

So, how do humans know about the existence of dark matter? As mentioned earlier, although dark matter has no electromagnetic force and does not interact with electromagnetic force, it has mass, and its mass is much greater than that of visible matter. The magnitude of gravity is proportional to mass, that is, the greater the mass, the greater the gravity. In this way, this invisible devil cannot hide.

This is a bit like a black hole. Due to the extremely strong gravitational force of a black hole, light that enters its sphere of influence (Schwarzschild radius) cannot escape, so black holes cannot be seen by humans. However, due to the extreme gravitational force of a black hole, any celestial matter that approaches a black hole will be torn apart and swallowed by it without mercy.

During this process, matter will be torn into pieces by the gravity of the black hole and rotate at high speed around the black hole's event horizon (that is, the critical point before falling into the Schwarzschild radius of the black hole). The matter that is torn into elementary particles will emit dazzling light and high-energy rays in the collision at speeds close to the speed of light, which can be observed by people.

Therefore, humans can indirectly "see" black holes.

The same is true for dark matter. Since it is widely present in interstellar space and has a large mass, the gravitational force it generates is very large, which will affect the surrounding space and time. We know that the movement of celestial bodies is governed by gravity, and their state of motion can be calculated by mass. If anomalies occur, either there is a problem with the theory of gravity, or there are things that have not yet been discovered that affect the movement of celestial bodies.

Many astronomers have found that the movement of some giant star clusters and other celestial bodies is inconsistent with the effects caused by visible mass. This phenomenon occurs randomly in all directions and distances in the universe, and it can be ruled out that there is a problem with the gravitational theory. Therefore, scientists tend to believe that there is invisible matter in the universe.

In 1922, Dutch astronomer Jacobs Kapteyn indirectly deduced the possible existence of invisible matter around stars by observing the movement of stars; in 1933, Swiss astronomer Fritz Zwicky used spectral redshift to measure the movement speed of each galaxy in the Coma Cluster relative to the galaxy cluster, and found that the galaxy dispersion was too high, and the gravitational force generated by the mass of visible matter in the galaxy cluster alone could not constrain these galaxies within the cluster.

From this, Zwicky deduced that there is a large amount of dark matter in galaxy clusters, whose mass is more than 100 times that of visible matter. Later, many scientists observed and verified such results, which indirectly verified the existence of dark matter.

Later, based on Einstein's general theory of relativity, people discovered that when light approaches or passes through a massive object, it will bend and magnify. He called this effect the gravitational lens effect and predicted the existence of such gravitational lenses in the universe.

People call this lens an Einstein ring.

In 1979, astronomers used the 2.1-meter telescope at Kitt Peak Observatory in the United States to observe for the first time a double image of a quasar formed by the gravitational lens effect; in 1998, the Hubble Telescope observed the first complete Einstein ring, named B1938+666; since then, gravitational lenses have been discovered continuously.

In 2007, the Hubble Space Telescope discovered an image of a dark matter ring in the CL0024+17 galaxy cluster with a diameter of 2.6 million light-years. This effect can only be achieved if there is a huge amount of dark matter in the galaxy cluster, which is more powerful indirect proof of the existence of dark matter.

Thanks to this gravitational lens magnification effect, humans have discovered stars and galaxies that are increasingly farther and farther away from us. For example, the farthest star discovered so far, WHL0137-LS, was born 900 million years after the Big Bang, and its light took 12.9 billion years to reach us.

This star, nicknamed "Morning Star", is visible to us because of the gravitational lensing effect of the huge galaxy cluster WHL0137-08. The gravitational lensing effect reaches 1,000 times, otherwise we cannot see the "Morning Star". The visible mass of this galaxy cluster alone cannot achieve such a huge lensing effect, thanks to the role of a large amount of dark matter in it.

What exactly is dark matter? Will we be able to figure it out in the future? The scientific community has not yet reached a conclusion on what dark matter is. Some people once suspected that neutrinos might be dark matter, but this was later denied. Although neutrinos basically do not interact with photons and have almost no electromagnetic force, they are not completely absent. There is still a tiny bit, one in 10 billion neutrinos will interact with matter, which leaves room for human detection.

Moreover, although there are many neutrinos in the universe, their mass share is very small, only 0.0034%. Neutrinos are produced in weak interactions. The nuclear reaction of the sun produces 10^38 neutrinos per second. Since they basically do not interact with matter, neutrinos come to the earth unimpeded at a speed close to the speed of light, and then pass through the earth to the distance.

Every second, day and night, about 100 trillion neutrinos from the sun pass through each human body, but people do not feel anything. However, since neutrinos still have some traces, scientists have already captured their traces and studied them more and more thoroughly. So, what other particles may be candidates for dark matter?

The scientific community focuses on massive particles in the weak interaction range, such as WIMP, including super neutralino, the smallest Kaluza-Klein excited state particle in the extra dimension theory, T-odd particle in the Little Higgs model, etc. There is also a very light neutral particle ~ axion, which is also a hot candidate.

But these particles still exist only in the realm of conjecture and have not been confirmed or discovered.

Now, scientists from all over the world are taking various approaches to try to find out the invisible devil, dark matter. The main methods are: 1. Direct detection, that is, through a three-dimensional detection system in the sky, underground, and deep in the ice, hoping to directly discover candidate particles captured by the detector; 2. Indirect detection, that is, looking for ordinary matter signals produced after the decay or annihilation of dark matter; 3. Artificial creation, that is, through equipment such as particle colliders, new particles are created to screen out dark matter.

It is foreseeable that the day when dark matter appears will be a major breakthrough in human understanding of the universe, and human civilization may be upgraded to a higher level. So can humans find dark matter as soon as possible, and how long will it take? Welcome to discuss, thank you for reading.

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