The problem of dark matter, or, as scientists more often call it, hidden mass, is one of the most intriguing mysteries of modern physics. It turns out that everything we see in the universe, everything that physical instruments can register, is only a small part of the existing nature. And it still remains to discover most of it. Kommersant-Nauka talks about all this with Konstantin Belotsky, a leading researcher and professor of the Department of Elementary Particle Physics at the National Research Nuclear University MEPhI, Doctor of Physico-Mathematical Sciences.

— So, what is a hidden mass?

— This is a kind of matter of unknown physical nature that fills our universe and which we know about by its gravitational role in the universe. First of all, it is the formation of the structure of galaxies. But not only the evidence that it has existed for a very long time. According to modern estimates, this matter is five times more than the ordinary matter known to us — the matter that you and I are made of.

— What are the main effects and phenomena that suggest that this matter exists, despite the fact that it is not visible?
— As I said, there are quite a lot of them. I'd count about ten of them. The most obvious among them are the rotation curves of galaxies. We see that the stars are rotating around the center of the galaxy at speeds much higher than they would have to be if this rotation were only due to ordinary matter. We know how the Earth revolves around the Sun. By the speed of the Earth's movement around the Sun, we can determine the mass of the Sun. In the same way, we can determine the mass of the galaxy by the speed of rotation of the stars in the galaxy. And we see that this mass is five times greater than the observed substance. And we observe this rotation even outside the visible galaxy. We can go beyond the disk of our galaxy, where we observe gas that is also rotating, and we see that its rotation corresponds to a much larger mass of the galaxy than the mass of the entire disk.

— Do these gravitational effects allow us to judge whether this hidden mass is evenly distributed in space or whether it is coalescing into some kind of "hidden planets"?
— That's a very good question. I would say that it is currently being worked out. That is, there is still no final opinion. Initially, it is believed that the hidden mass forms only the structure of the Universe on the scale of galaxies. And ordinary matter seems to fall into this structure. That is, the hidden mass forms a general structure, and then we sort of get into this structure, because there is five times more of it, and it dictates to us, that is, visible matter, the general laws of spatial distribution. But whether it can form smaller structures is all up for discussion. There is even a hypothesis that it can form "dark stars". That is, such inconspicuous and very compact "lumps" of dark matter.

— Is there a hidden mass on structures larger than galaxies? For example, in clusters of galaxies?

— A cluster of galaxies is formed exclusively by a hidden mass. Here's the ordinary matter that you and I are made of—she wouldn't be able to do that. She kind of plays a secondary role here. And this is one of the main evidences that dark matter exists.

— So, the main "evidence" of the existence of a hidden mass is the rotation speed of space objects?

— Another type of evidence is related to potential energy. In galaxy clusters and inside galaxies, gas accumulates, which takes on a temperature corresponding to potential energy. That is, we can measure the potential energy of a galaxy or a cluster of galaxies by the temperature of the gas. And we've known since school that the potential gravitational energy is determined by the total mass. And we see that the temperature of the gas corresponds to a much larger mass than we observe. Another independent type of evidence of hidden mass is cosmic microwave background radiation, which is called "cosmic microwave radiation" in English. It is known to give an imprint of the composition of the universe from the very beginning, when there were not even galaxies yet. Nevertheless, the hidden mass produced some disturbances in this cosmic microwave background radiation, some inhomogeneities. They are very small, but they are nevertheless detected by modern devices. And we see that, judging by these perturbations, there was much more hidden mass than ordinary matter. And here it is necessary to superimpose this evidence on calculations according to which, without hidden mass, we would not have been able to form such galaxies by this epoch as we are now. If the galaxies consisted only of ordinary matter, of which you and I are composed, it would take much longer.

— How much more?

— About ten times more. And we see that the galaxies have already formed as they are.

— What are the main ways that science is currently looking for dark matter?

— There are many directions here. First, direct searches. We understand that there is a hidden mass around us. Even in this room. We're talking right now, and she's here. And we assume that it exists in the form of some kind of particles. This means that these particles can be detected by detectors.

— So you're suggesting that these invisible particles interact with visible particles in some way?
— That's a very good question. We hope so. In general, all hypotheses about the nature of the hidden mass can be divided into two classes. The first class is that they interact with us in different ways, the second class is that they only interact gravitationally. I'm sticking to the hypothesis that they interact through different types of interaction. The class of hypotheses that they only interact gravitationally includes, first of all, the hypothesis that the hidden mass consists of so-called primary black holes and, perhaps, some other gravitational defects in space-time.

— It sounds mysterious…

— It's hard to explain at all. Our space is three-dimensional, and in these hypotheses it is assumed that space can have more than three dimensions. And we have knots of extra dimensions forming in our space. Imagine that you are knitting a space of three dimensions with knitting needles, and you are folding the extra dimensions into knots. And these nodules can manifest themselves in our dimension as a hidden mass. However, the additional dimensions may be small. We know that our three-dimensional space is expanding. But maybe that didn't happen with the extra dimensions, maybe they stayed small and curled up into some kind of knots.

— Or maybe they are a reserve, due to which our space is expanding?

— These are all very difficult questions. This is the cutting edge of science. And so far it is assumed that we cannot look into these nodules. Because our technical capabilities do not allow us to look into objects of such a small size. To explore such a small size, we need high energies. Accelerating energies may not be enough. Nevertheless, additional dimensions can exist in the form of compactified nodules, and we feel them as a hidden mass. In general, these hypotheses run into the problem of constructing a theory of quantum gravity. It has been a dream since Einstein's time to build a theory that includes a description of all interactions, including gravity. But such a theory has not yet been built.

— The second class of hypotheses, how can you understand, is more mundane?

— Yes, it is more traditional: that the hidden mass consists of particles that simply interact poorly with us. I believe that they interact with us not only gravitationally. In general, everything that exists in nature necessarily interacts with us gravitationally. It doesn't happen that something doesn't interact. If something doesn't interact gravitationally, it means it doesn't exist in

nature.

— And the light? It has no mass.

— Nevertheless, it also interacts gravitationally. It is a proven fact that a ray of light is deflected in a gravitational field. This is the classical theory of gravity created by Einstein, which says that gravity describes the geometry of space. Therefore, everything that exists in space interacts gravitationally. And this second class of hypotheses suggests that the hidden mass consists of approximately the same particles that we are made of, but others of the same type. And they should enter into other interactions with us, not only gravitational ones.

— Why?

— If they only interact gravitationally, then this is such a boring case. And I believe that it is not being implemented. The particles must interact with us in some other way. Because if they only interact gravitationally, then they have no meaning. We must somehow fit them into the overall picture of the world. If we have a new particle, what is its role in the general theory? It must be inserted somewhere. Imagine the periodic table. Can you put an element in there that doesn't consist of protons and neutrons? No. It can't be done. It's the same here. If you want to include a particle in our "table" that does not have any interaction other than gravitational, then there will be no place for it.

— Are there any assumptions about the properties of these particles?

— This is a completely open question.

— And what is the concept of the "dark atom"?

— This is one of the existing hypotheses. For example, it is put forward by Maxim Yuryevich Khlopov, a professor at MEPhI. He explores the hypothesis that the hidden mass may consist of our particles. It is assumed that there is some unknown particle X with a negative electric charge -2, that is, a charge equal to two charges of an electron. And this particle X combines with the helium nucleus. The helium nucleus has a charge of +2, and it has two protons. As a result of this connection, we get an electroneutral particle. That is, an X with a charge of -2 combines with a helium nucleus with a charge of +2, and a "dark atom" is obtained. It's very small. It is the size of a helium nucleus. It is electroneutral. In a sense, it turns out to be such a superheavy neutron. And this does not contradict the observations.

— And such a "dark atom" should be invisible?

— Yes, it is invisible because it is electroneutral. Therefore, it is not so easy to detect it. In general, you can try to find it, and colleagues are investigating all the experiments that are currently being conducted to find the hidden mass. And they say that the "dark atom" is invisible, it bypasses all observational data. This is a very exotic hypothesis.

— In your opinion, of the experiments being conducted today or, perhaps, space missions, which are the most promising from the point of view of searching for hidden mass?

— There are several research directions. There are underground experiments. We have hidden mass everywhere, and we're going to put a detector here and look for it. Of course, these detectors are not placed in a room, but underground, so that there is less distortion. The second area of research is accelerators, in which we try to "give birth" to particles of hidden mass and register them there. And the third direction is indirect manifestations, in particular in cosmic rays, and other cosmic manifestations. All these studies are based on the hope that the hidden mass has some kind of interaction, so that it can turn into our observed particles, and these may be some kind of bursts in cosmic rays.

— Are there any hypotheses about the connection of hidden mass with dark energy?
—You've got me right stumped." There is no such connection. These are two separate issues.