Which of the following best summarizes what we mean by dark matter?
Dark matter is a mysterious substance that can be indirectly inferred to exist in the universe through gravity, but cannot be observed by any other means, including electromagnetic waves.
Various hypotheses have been proposed, such as primordial black holes, inactive neutrinos, or even mass of quarks that remain unatomized, such as the result of missing from the total mass of dark objects such as brown dwarfs and inactive black holes that have not been discovered by mankind. Because each of them has loopholes, the identity has not been revealed yet.
A gravitational field can only be created by particles with mass. For this reason, when matter and electromagnetic waves interact with the gravitational field, a corresponding amount of mass must exist at the center of the gravitational field.
However, in the real universe, it is very frequent that sufficient mass is not detected to explain the magnitude of the measured gravitational force, and this discrepancy appears as if some kind of unobservable material is very widespread in the universe.
To explain this phenomenon, the concept of dark matter was proposed. The name ‘darkness’ is a temporary name because it has not been observed by any means other than gravity and its identity is still unknown. Scholars who are cautious about whether dark matter actually exists have called it the Missing Mass Problem.
One might think that the observed mass was simply measured incorrectly, but the problem is that there are an unusually large number of unobserved masses. The total amount of matter that should exist in the universe, estimated through gravity, is six times that of general matter estimated through electromagnetic wave observation.
In other words, the amount of dark matter distributed in the universe is much greater than that of observable matter. It is estimated that galaxies such as our own Milky Way and the Andromeda Galaxy are surrounded by dark matter tens to hundreds of times their own mass, and it is known that the larger the galaxy, the more dark matter.
However, it is not absolutely proportional, and the Andromeda Galaxy has more than twice as many stars as our Milky Way, but it is estimated that the masses are similar because our Milky Way contains much more dark matter.
Dark matter is speculated to have different properties from ordinary matter as we know it. Of course, matter composed of ordinary particles can fall into the category of dark matter even if it is simply not observed.
In fact, the universe is rich in hydrogen and dust, with the exception of luminous stars. Most of these interstellar materials emit very little visible light, so they cannot be seen with the naked eye, but can be detected indirectly through other wavelengths, such as radio waves or X-rays, or through effects that obscure light from the background, such as interstellar extinction/redness is possible.
However, dark matter composed of baryons, if any, would be only a fraction of the total dark matter. The dark matter that scientists currently assume is a completely separate matter from them. Dark matter emits little or no light or particles at any wavelength and does not interact with other particles, so it is impossible to detect except for gravity and gravitational lensing caused by it.
Although it has not yet been detected, it is highly likely that small amounts of dark matter particles are floating in the solar system or the space around the Earth. It may even be that dark matter is passing through your body right now.
Does dark matter exist?
The fact that a clear identity, nature, and origin have not yet been established has made many people doubt the existence of dark matter. Since the concept of dark matter in the first place is an idea like Deus ex Machina inserted by scientists to make up for something that cannot be explained by the formulas they know, there are still opinions that feel uncomfortable with the concept itself.
Moreover, since dark matter and normal matter alone cannot explain the accelerated expansion of the universe, the concept of dark energy may have to be reintroduced.
Dark matter and dark energy, which do not interact with ordinary matter and cannot even be observed, are only hypotheses that plausibly fit the theory into a phenomenon that goes against the existing laws are doing Without breaking the law of conservation of mass, the introduction of (positive) phlogiston to account for the decrease in mass during combustion of organic matter and negative phlogiston to account for the increase in mass during the combustion of metals is the same as the introduction of dark matter and dark energy.
There is an opinion that humans are repeating the same mistakes in the 21st century as they did in the 17th century because they are so structurally similar.
However, it can be said that the mainstream theory of academia always chooses the most conservative, and the dark matter and dark energy hypotheses are accepted because they have passed rigorous tests by the academic community.
Just as the phlogiston hypothesis was an inevitable transitional prescription to explain a new phenomenon while maintaining the theory at the time. At least, it is an empirical law that does not conflict with other laws currently known and explains the phenomenon well.
In summary, regardless of what dark matter is, it is self-evident that there is ‘something’ that we do not know. It could be a really new substance, or it could be a new behavior of a substance we already knew, or it could even be something we were familiar with but no one predicted. It’s just that humans aren’t smart enough to figure this out yet.
Ether and phlogiston, which were found not to exist for granted now, had no explanation at that time either, and all scientists believed that it was the truth, but as science developed, it ended up as just a hypothesis. There is no guarantee that dark matter will not.
Since the evidence of additional gravity that cannot be explained by the observable mass of galaxies is so clear, even those who deny the existence of dark matter do not deny these observations themselves, but try to solve the mass disappearance problem by modifying the existing gravitational theory.
They are trying, but their progress has not progressed as much as attempts to observe dark matter. Since touching gravity means reconstructing the theory of relativity, one of the great pillars of modern physics, from the ground up, we have to overcome a lot of obstacles.
In the current situation where attempts to deny dark matter continue to fail and remain a fringe theory, the mainstream physics and astronomy circles up to now have only disagreements about its characteristics or identity, and generally cannot assume the existence of dark matter. I think the reasons are good enough.
The basis of dark matter
Galactic Rotation Curve Since the study of galaxy rotation curves and clusters in the 1930s, research results in various fields support the existence of dark matter.
1. Galaxy rotation curve
Unexpected rotational speeds around the galactic periphery were first reported by Babcock in 1939. When observing the motion of a celestial body revolving around a specific center of mass, it should be observed that the orbital speed slows as the distance from the center increases according to Kepler’s law.
If it is far from the center, the centripetal force (gravity) is weak and the speed is small, but if it is close to the center, the centripetal force (gravity) is strong and the speed is high. For example, Mercury, the closest planet to the Sun in the solar system, has the fastest orbits and Neptune has the slowest orbits.
The structure of the Milky Way also increases the density of stars toward the center, and most of the observable mass is distributed inside the orbit of the Sun. That is, it is predicted that the orbital speed of stars will decrease according to Kepler’s law as they go outward from the orbit of the sun from the center of the galaxy.
By the 1980s, large optical telescopes and advanced radio telescope technology made it possible to accurately calculate orbital speeds by analyzing the actual motions of stars orbiting our galaxy. Then, conclusive evidence began to emerge that the stars in the outer galaxies orbit the Sun at or near the speed of the Sun.
This is a phenomenon that cannot be explained unless there is a large amount of invisible mass on the outskirts of our galaxy that can overwhelm the mass of stars. Conversely, if there is no dark matter surrounding our galaxy, the stars outside the galaxy must be scattered by their own orbital speed.
This reversal of orbital speed is observed in most galaxies of all types, which means that most galaxies in the universe are much heavier than themselves and are surrounded by a large dark halo.
2. Mass of galaxies
The possibility of dark matter in galaxy clusters was first proposed by Swiss astronomer Fritz Zwicky in 1933. The singular velocities of galaxies located at the center of clusters are nearly 1000 km/s.
This means that an object with a mass high enough to trap these fast-flying galaxies must be located at the center of the cluster. However, the total amount of observable stars and gases in the cluster is insufficient to account for this mass.
Moreover, a strong gravitational lensing effect occurs in large clusters of galaxies, which is also impossible with the mass of galaxies alone. The total mass of galaxy clusters calculated through optical observation is about 1 to 10 trillion times that of the Sun, whereas the kinematic mass is often easily over 100 trillion times that of the Sun.
The fact that the cluster’s actual mass is much larger than the visible portion means that it must contain a huge amount of dark matter. If there was no dark matter, the galaxies that make up the cluster would soon have to disperse due to their high orbital speed. Like galaxies, dark matter is thought to surround clusters in halos over a very wide range.
The bullet cluster shown in the image above shows just after the two clusters collide head-on. The distribution of galaxies forms two large clusters on both sides, whereas the distribution of gases in the cluster, marked in red, does not follow the motion of galaxies and remains at the collision point. This means that the stars constituting the cluster collide
This is because the area is so small that they pass through each other while the hot gases collide and mix with each other. The blue color is the distribution of mass obtained through gravitational lensing, and it can be seen that it is consistent with the distribution of galaxies, not gases.
Considering that most of the normal matter constituting galaxy clusters consists of non-star gas, it cannot be explained without the existence of dark matter. From this it can be inferred that most of the cluster mass is composed of unusual material, with no or extremely small impact area.
3. Cosmic Megastructure
After the fact that there were countless galaxies outside the Milky Way in 1924, it was discovered in the mid-1980s that galaxies were not evenly distributed in the universe.
Numerous astronomers with doubts about this have been engaged in research, and Margaret J. Discovered (1989) the Great Wall, an interconnecting wall-like structure. It has also been discovered that galaxies have small regions of voids. Subsequent observations have shown that galaxies are distributed in a filamentous structure similar to bubbles, spanning hundreds of millions of light-years.
At the point where the filaments meet, a cluster of galaxies is located. The distribution pattern of these galaxies, called the cosmic giant structure, is only visible when a map is drawn based on the positions of hundreds of thousands of galaxies. It was difficult to do. In other words, there is a need for dark matter that can provide additional gravity without being observed.
In modern times, the computational power of computers has increased, making it possible to simulate the gravity of the entire universe. As a result of millennium simulations  conducted in 2000, predictions emerged that dark matter would form a filamentous structure similar to that of observed galaxies.
In other words, the distribution of galaxies, known as the cosmic giant structure, in the form of galaxies similar to filaments, actually represents the distribution of the underlying invisible matter. It can be said that galaxies were born by being naturally drawn to the place.
4. Big Bang Nuclear Fusion
The mass ratio of hydrogen to helium currently found in the universe is about 3:1. Helium, which accounts for one of them, was born from nuclear fusion that occurred in the hot universe at the time of the Big Bang, mostly unrelated to stellar nuclear fusion.
For this ratio to come out, the ratio of baryons, that is, general matter, in the universe at the time of the Big Bang must be about 5%. However, the proportion of actual matter revealed through studies such as cosmic background radiation and type Ia supernovae is about 30%. This means that 5/6 of the mass in the universe is made up of unusual materials that are not involved in nuclear fusion at all.
Galaxies with little Dark Matter
Paradoxically, galaxies with little dark matter are also support for the dark matter theory. With this discovery, alternative theories of dark matter, such as modified Newtonian mechanics, were severely damaged.
In modified Newtonian mechanics, instead of introducing a new mass called dark matter, the gravitational force itself is stronger at a galaxy-scale distance, and attempts to explain the orbital velocity of objects within the galaxy.
In this case, it does not explain the case where the orbital speed is much slower than that of other similar galaxies, that is, the magnitude of the gravitational force is arbitrarily small. On the other hand, in the explanation using dark matter, the difference in orbital speed can be explained by the difference in the amount of dark matter.
Discovered in 2015, NGC 1052-DF2 has been shown to be a galaxy with little dark matter compared to other galaxies. The reason why dark matter was first proposed was because the orbital speed of the celestial bodies inside the galaxy was faster than expected.
Invisible masses were taken into account because the combined mass of those observed in galaxies could not hold the celestial body to orbit at that speed. However, the speed of the celestial bodies inside this galaxy is slow, so even a relatively small amount of dark matter could explain the orbital speed well. This discovery needed to revise the theory that galaxies were formed from abundant dark matter.
However, there are criticisms that a sufficient level of confidence in this discovery is not secured because the number of globular clusters used to measure the mass is only 10.
Moreover, there are many views that question this discovery because if these types of galaxies are continuously discovered, it could damage not only MOND, but also the currently accepted standard model of the universe.
And it was announced that there was an error in the actual measurement.
In 2019, Yale University discovered another dark matter-poor galaxy, NGC 1052-DF4, but this time around, it is suspected that the distance measurement was wrong.
Predicting the future of the Universe
When the outer space exceeds a certain level of density, the attraction between matter becomes stronger from that point on and contracts. In this state, all matter gathers at one point and returns to the high-temperature universe as Big Crunch.
When the density of outer space is below a certain level, space continues to expand and converge to absolute zero. When space is ripped apart atom by atom due to excessive expansion forces, it is called the Big Rip.
Dark matter, which accounts for most of the mass of matter in the universe, exerts a gravitational force on each other to suppress the expansion of the universe. In the past, it was thought that by finding out the mass of matter in the entire universe, it would be possible to determine the present age of the universe and, furthermore, to predict the fate of the universe.
However, as the existence of dark energy, a factor that accelerates the expansion of the universe, has been revealed, it is now possible to predict the future of the universe only if both dark matter and the amount of dark energy are accurately known. The revealed dark matter to dark energy ratio is about 3:7, and the universe is expected to end in a big freeze or big rip.
Which of the following are candidates for Dark Matter?
Initially, the fast-moving neutrino was the strongest candidate for dark matter as hot dark matter (HDM), but after running simulations, it was found that the current galaxies could not be formed because of their fast-moving galaxies. .
Since then, medium-sized elementary particles (WDM, Warm Dark Matter) and heavy and slow elementary particles (CDM, Cold Dark Matter) are currently strong candidates.
Neutrinos: Their mass or quantity was not precisely measured at the beginning of their discovery, and they were chosen as strong candidates for dark matter due to their characteristics that they rarely interact with other matter.
However, since it moves at a speed close to the speed of light, if dark matter has the characteristics of a neutrino, there is a problem that existing galaxies halos or large structures in the universe are not formed at all, so it was excluded from candidates for explaining most of dark matter.
However, since there is still a possibility that neutrinos occupy a part of dark matter, a form of dark matter mixed with other types of particles (MDM) is being studied.
WIMPs: Weakly Interacting Massive Particles. Weakly interacting heavy particles. The meaning of wimp is a coward, and this is a very intentional name. Of the basic interactions, except for gravity, the most likely way dark matter reacts is weak interactions.
In addition, the fact that dark matter with a weak interaction mass (about 100 GeV) and binding constant (Fermi constant) coincides with the freeze-out process, which is the most natural dark matter generation process, is known as a WIMP miracle, which is very popular.
They must be large enough to account for the vast amount of dark matter, with little interaction with other matter, such as neutrinos. In the case of neutrinos, they are eliminated from the dark matter candidates because they move close to the speed of light.
Lightest Supersymmetric Particle (LSP): A particle derived from the theory of supersymmetry.
Gravitino: A super-pair particle of a gravitational force. A promising candidate for dark matter.
Lightest Kaluza-Klein particle (LKP): A particle derived from the universal extra dimension theory. It is also derived from the warped extra dimension theory, a theory that adds curvature to the extra dimension.
Lightest T-odd particle (LTP): A particle derived from Little Higgs theory.
Lightest Inert Particle (LIP): A particle derived from the inert doubldet model, a theory created by introducing a scalar field with properties similar to the Higgs field.
Technibaryon: Derived from the technicolor theory, a theory of symmetrical collapse of weak force. Depending on the details of the theory, both symmetric dark matter by thermal freezing and asymmetric dark matter by a method similar to baryogenesis are possible.
Q-ball: Derived from the theory of supersymmetry.
Abbreviation for MAssive Compact Halo Objects. When particle physicists announced wimps, astrophysicists came up with the opposite forage and coined the name.
Examples of such objects include inactive black holes, neutron stars, brown dwarfs (a mass of gas that does not become a star and emits almost no light), red dwarfs, and wandering planets. Except for primordial black holes, these are celestial bodies created through barion matter, including nucleons.
Because it is predicted to have significant mass and is not well observed, it was considered as one of the candidates, but a series of subsequent studies concluded that the amount is insufficient to fully explain dark matter.
Typically, the density of barion matter matching the current cosmic hydrogen/helium ratio was too low, and in particular, the frequency of microgravity lensing caused by MACHO in our galaxy was too low.
Primordial black hole: A black hole that formed very early in the universe and has not yet been identified. This explanation of dark matter requires an initial inhomogeneity greater than that of standard cosmology. Much was ruled out by MACHO gravitational lensing or other physical factors. However, the possibility remains open within some mass range.
Quark nugget: A substance with a strong concentration of quarks. Depending on the mass, it is also included in MACHO. It is possible that the universe was created through a QCD phase transition.
It is not counted as a barion material because it was not in the form of a gas at the time of the Big Bang nucleosynthesis or formation of the cosmic background radiation. The strange body is a type of quark nugget.
AXION: It does not collide with particles and is denser than it appears. Although the mass is small, it is not created by thermal freezing, but by a separate mechanism, thus satisfying the condition of dark matter.
Sterile neutrino: A heavy, inactive neutrino. derived from the seesaw mechanism.
Dark photon: A photon with mass. It seems contradictory that a photon, the representative particle of luxon with a rest mass of 0, has mass, but it is precisely a gauge boson with mass that appears in the expansion theory, which newly added U(1) symmetry to the symmetry group of the standard model.
Theoretically, a photon becomes a dark photon or a dark photon becomes a photon because it is possible to convert between photons and photons through U(1) transformation. With this in mind, there are attempts to detect dark photons. Also, since it is a gauge boson, it is presumed to be a mediator of another unknown force (aka dark charge).
Theoretically, it is assumed that dark photons exist, and unknown quarks exist, and these quarks are said to be the mediator of generating dark charges, so these quarks and dark photons interact, and this interaction is also directly related to the axion existing above.