Astrophysicists Solve Mystery of How Dark Matter Is Distributed in Galaxies

Dark Matter in Two Galaxies

Dark matter in two galaxies simulated on a computer. The only difference between them is the nature of dark matter. Without collisions on the left and with collisions on the right. The work suggests that dark matter in real galaxies looks more like the image on the right, less clumpy, and more diffuse than the one on the left. The circle marks the end of the galaxy. Credit: Image taken from the article Brinckmann et al. (2018, Monthly Notices of the Royal Astronomical Society, 474, 746;

The gravitational force in the Universe under which it has evolved from a state almost uniform at the Big Bang until now, when matter is concentrated in galaxies, stars, and planets, is provided by what is termed ‘dark matter.’ But in spite of the essential role that this extra material plays, we know almost nothing about its nature, behavior, and composition, which is one of the basic problems of modern physics. In a recent article in Astronomy & Astrophysics Letters, scientists at the Instituto de Astrofísica de Canarias (IAC)/University of La Laguna (ULL) and of the National University of the North-West of the Province of Buenos Aires (Junín, Argentina) have shown that the dark matter in galaxies follows a ‘maximum entropy’ distribution, which sheds light on its nature.

Dark matter makes up 85% of the matter of the Universe, but its existence shows up only on astronomical scales. That is to say, due to its weak interaction, the net effect can only be noticed when it is present in huge quantities. As it cools down only with difficulty, the structures it forms are generally much bigger than planets and stars. As the presence of dark matter shows up only on large scales the discovery of its nature probably has to be made by astrophysical studies.

Maximum Entropy

To say that the distribution of dark matter is organized according to maximum entropy (which is equivalent to ‘maximum disorder’ or ‘thermodynamic equilibrium’) means that it is found in its most probable state. To reach this ‘maximum disorder’ the dark matter must have had to collide within itself, just as gas molecules do, so as to reach an equilibrium in which its density, pressure, and temperature are related. However, we do not know how the dark matter has reached this type of equilibrium.

“Unlike the molecules in the air, for example, because gravitational action is weak, dark matter particles ought hardly to collide with one another, so that the mechanism by which they reach equilibrium is a mystery,” says Jorge Sánchez Almeida, an IAC researcher who is the first author of the article. “However if they did collide with one another this would give them a very special nature, which would partly solve the mystery of their origin,” he adds.

The maximum entropy of dark matter has been detected in dwarf galaxies, which have a higher ratio of dark matter to total matter than more massive galaxies, so it is easier to see the effect in them. However, the researchers expect that it is general behavior in all types of galaxies.

The study implies that the distribution of matter in thermodynamic equilibrium has a much lower central density than astronomers have assumed for many practical applications, such as in the correct interpretation of gravitational lenses, or when designing experiments to detect dark matter by its self-annihilation.

This central density is basic for the correct interpretation of the curvature of the light by gravitational lenses: if it is less dense the effect of the lens is less. To use a gravitational lens to measure the mass of a galaxy one needs a model, if this model is changed, the measurement changes.

The central density also is very important for the experiments which try to detect dark matter using its self-annihilation. Two dark matter particles could interact and disappear in a process that is highly improbable, but which would be characteristic of their nature. For two particles to interact they must collide. The probability of this collision depends on the density of the dark matter; the higher the concentration of dark matter, the higher is the probability that the particles will collide.

“For that reason, if the density changes so will the expected rate of production of the self-annihilations, and given that the experiments are designed on the prediction of a given rate, if this rate were very low the experiment is unlikely to yield a positive result,” says Sánchez Almeida.

Finally, thermodynamic equilibrium for dark matter could also explain the brightness profile of the galaxies. This brightness falls with distance from the center of a galaxy in a specific way, whose physical origin is unknown, but for which the researchers are working to show that it is the result of an equilibrium with maximum entropy.

Simulation Versus Observation

The density of dark matter in the centers of galaxies has been a mystery for decades. There is a strong discrepancy between the predictions of the simulations (a high density) and that which is observed (a low value). Astronomers have put forward many types of mechanisms to resolve this major disagreement.

In this article, the researchers have shown, using basic physical principles, that the observations can be reproduced on the assumption that the dark matter is in equilibrium, i.e., that it has maximum entropy. The consequences of this result could be very important because they indicate that the dark matter has interchanged energy with itself and/or with the remaining “normal” (baryonic) matter.

“The fact that equilibrium has been reached in such a short time, compared with the age of the Universe, could be the result of a type of interaction between dark matter and normal matter in addition to gravity,” suggests Ignacio Trujillo, an IAC researcher and a co-author of this article. “The exact nature of this mechanism needs to be explored, but the consequences could be fascinating to understand just what is this component which dominates the total amount of matter in the Universe.”

Reference: “The principle of maximum entropy explains the cores observed in the mass distribution of dwarf galaxies” by Jorge Sánchez Almeida, Ignacio Trujillo and Angel Ricardo Plastino, 12 October 2020, Astronomy & Astrophysics Letters.
DOI: 10.1051/0004-6361/202039190

29 Comments on "Astrophysicists Solve Mystery of How Dark Matter Is Distributed in Galaxies"

  1. “The gravitational force in the Universe under which it has evolved from a state almost uniform at the Big Bang until now, when matter is concentrated in galaxies, stars and planets, is provided by what is termed ‘dark matter.’ ”
    Word salad. Gave up after first paragraph.

    • Torbjörn Larsson | November 3, 2020 at 1:29 pm | Reply

      It’s a very pity description of current inflationary hot big bang cosmology, I’ll give you that. The initial universe is homogeneous and isotropic to 1 part in 100,000 as seen in the cosmic background radiation spectra, carrying over an imprint of energy density fluctuations of the inflation quantum field to gas density and velocity fluctuations at the start of the hot big bang. Those 3D gas fluctuations comes out analogous to how 2D light caustics at the bottom of a swimming pool come out from wind disturbances at its surface – they built the cosmic web of filaments.

      Gravitation tightened up the web as the gas started to flow towards the web nodes while building galaxies and galaxy clusters. Dark matter is dominating at 5 times the mass of normal matter (and one can see from models that without it there would be very few galaxies).

  2. Helium 4 from Baryonic dark matter ? Wiki

    If helium 3 and 4 can escape most gravitational fields – why couldn’t the composition of dark matter in the universe be helium 3 or 4 gravity particles ?

    Dear DOE

    Baryonic dark matter conversion of matter to helium-4 ? see

    Baryonic dark matter

    Big Bang nucleosynthesis

    Helium 4 from Baryonic dark matter ? Wiki Oct 12th 2020

    Big Bang nucleosynthesis
    From the perspective of Big Bang nucleosynthesis, a larger amount of ordinary (baryonic) matter implies a denser early universe, more efficient conversion of matter to helium-4, and less unburned deuterium remaining. If all of the dark matter in the universe were baryonic, then there would be much less deuterium in the universe than is observed.

    If this is the case the universe is much older than 60 Billion years .

    If you can verify that dark matter is helium 3 or 4 gravity then maybe you’ll win a prize.

    Why is 23% of the Helium in the universe unexplained?

    23% of the Helium in the universe being unexplained…Helium 4 …as I stated Helium 3 and 4 can not freeze even at the alleged absolute ZERO Kelvin if that was even possible.

    please read

    ” All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. Helium-4, by contrast, makes up about 23% of the universe’s ordinary matter—nearly all the ordinary matter that is not hydrogen. ”

    Helium – › wiki › Helium

    I hope this clarifies the 23% of the helium in the universe story I read…it is helium 4 that accounts for 23% of the helium in the universe and its formation is not explained as of yet ?

    If this is true that 23% of the Helium in the universe is unexplained , there is a little known fact that Helium 3 and 4 can not freeze even at an alleged 0 Kelvin temperature.

    As I have been trying to understand how and when and where motion first started in the universe to me it is apparent that this 23% of the unexplained helium in the universe is left over from a past cycle of expanding and decay of galaxies , leaving helium 3 and 4 and gravity …meaning the universe is much older than 13.8 Billion years ( my guess is the universe is in its forth cycle of decay and is about 4 x 13.8 billion yo or about 60 billion yo ) and that motion has been existence in the universe even before that, because if everything decayed to absolute ZERO Kelvin there would be no motion – but the universe allegedly cant go back to a motionless state now, because Helium 4 can not freeze this saving the day of a motionless static universe in the future.

    source of Helium information ;

    Neutroid Steady State Galaxy Theory


    ” Basic Operation of Galaxies

    ( this theory in error ? claims galaxies do not decay but are recycled through process )

    ” At the center of each galaxy is a neutroid which acts to constantly recycle all the matter and energy in the galaxy. This neutroid is similar to a neutron star but is very much larger and has reached a size where the pressure and temperature at its surface are great enough to generate a nuclear fusion process. In the areas of the neutroid’s magnetic poles, the products of fusion are trapped by the magnetic field and are pushed out along the magnetic field by the pressure of the nuclear fusion process going on below. This results in a column of material composed of hydrogen, helium and other light elements being ejected at each of the neutroid’s two magnetic poles. This material moves out from the neutroid at essentially constant velocity until it reaches a point where the magnetic field is no longer strong enough to control it. Once free of the magnetic field the material then continues under it’s own momentum to travel to the outer edge of the galaxy before starting to fall back toward the neutroid. …..


    A fourth arguement which has been used to support the Big Bang theory is that it would account for the abundance of helium we find in the universe. The amount of helium present (24%) cannot be accounted for by star production and according to Gamow it was generated by the Big Bang.

    Under the Steady State Galaxy theory, the nuclear fusion process which is expelling the material from the neutroid would generate large amounts of helium as well as other light elements and is the source of the excess helium found in the universe.”

    If helium 4 can escape most gravitational fields – why couldn’t the composition of dark matter in the universe be helium 4 gravity particles? If this is the case the universe is much older than 60 Billion years .

    If you can verify that dark matter is helium 3 or 4 gravity then you’ll win a prize.

    • Torbjörn Larsson | November 3, 2020 at 1:30 pm | Reply

      I wonder if a bot is trolling this pseudoscience with links and all? It is virtually the same comment repeatedly pasted on science sites.

  3. I wonder if the quantum properties of matter are imparted in some similar way that the Higgs boson imparts mass. Perhaps there is a non-failsafe mechanism for this; i.e., parts of the impartation fail and leave what should have been ordinary matter dark.

    • Torbjörn Larsson | November 3, 2020 at 1:36 pm | Reply

      But you already have normal matter mass from Higgs.

      And more, the Higgs mechanism only couples to normal matter [“Mathematical formulation of the Standard Model” @ Wikipedia].

      And more more, the Higgs field(s)* are a scalar field.

      *See the reference for why there are 4 Higgs fields.

  4. Maybe dark matter is just the field of matter that sits in gravitational waves as an anomaly? Think about how a magnet works. The solid magnet is the matter but the magnetic field it creates is the antimatter that we can’t see. However, we can measure and see the field using tiny fragments of metal. Therefore, the shape of celestial bodies such as galaxy formation (spiral galaxy) shows us dark matter (anti matter) as a visual entity.

    • Torbjörn Larsson | November 3, 2020 at 1:45 pm | Reply

      Dark matter is a particulate gas, not a field. The latter was rejected by observations long ago [Planck collaboration cosmological summary paper @ Planck Legacy Archive; eBOSS collaboration 20 year cosmological summary paper @ arxiv].

      And it isn’t related to normal matter either, so isn’t antimatter, as can easily be seen by anyone in the cosmic background spectra: “The genome of the universe” @ Galileo’s Pendulum; “Secrets of the cosmic microwave background” @ PBS Space Time.

  5. … and some people say, that the dark matter is where it is needed in a calculations. In short, a good cooking of a book keeping,…

  6. The article states that dark matter distributed through out the galaxy is at maximum entropy? How can that be if it is matter and has the ability to be detected by its interaction with gravity does that not at least give it potential energy status?

    • Torbjörn Larsson | November 3, 2020 at 1:55 pm | Reply

      Maximum entropy is an information theoretic concept – used to establish the self-interacting dark matter model – see my longer comment.

      Entropy isn’t to be confused with energy in thermodynamics. On macroscale it is a characteristic of heat Q under small changes in temperature dQ = SdT. On microscale it is an enumeration of the number of microstates at the same energy. And it just tends towards larger values in closed systems according to the 2nd law.

      Besides the possible nitpicks, you are precisely morally correct – dark matter seems distributed according to gravity, not gravity and self-interactions.

      • Torbjörn Larsson | November 3, 2020 at 1:57 pm | Reply

        “On microscale it is an enumeration of the number of microstates at the same energy” = On microscale it is an enumeration of the number of accessed microstates at the same energy. [See a textbook in Statistical Physics.]

  7. Torbjörn Larsson | November 3, 2020 at 1:20 pm | Reply

    This Self-interacting dark matter model [“Self-interacting dark matter” @ Wikipedia] is literary a shot in the dark, of course.

    The current Cold Dark Matter model explains the observations better than other dark matter models, and the cusp-core problem the paper alludes to was solved 2015 by including galaxy gas outflows [“The Labor of Outflows against Dark Matter Halo” @ astrobitesDOTorg]

    “Alternative dark matter models such as warm dark matter (WDM) and self-interating dark matter (SIDM) are introduced to resolve the cusp-core problem. However, ?CDM cosmology may still give rise to cored halos, by utilizing astrophysical processes that “push against” dark matter halos such that they are no longer steep and cuspy in the centers. One such process is outflow from star forming activities such as supernovae. … the [CDM] DC14 model closely tracks observations … both WDM and SIDM are not able to fit the velocity function and Tully-Fisher relation as well as we expect.”

    I don’t see that the paper tries to test their cored model against the velocity function and Tully-Fisher relation.

    Also, maximum entropy is an information theoretic concept, not a physically motivated result [“Principle of maximum entropy” @ Wikipedia].

  8. Allen L. Princr | November 12, 2020 at 3:18 pm | Reply

    On dark matter, I published books, “dark matter mechanics”,I argue density variations of vacuum that allow forces that on a cosmic scale appears as a different substance compared empty space when more dense then vacuum elsewhere. This is eunique concept based on gradient vacuum cause gravity and other forces in vacuum

  9. checker? L. Prince | November 12, 2020 at 3:21 pm | Reply

    Where’s the spell hcecker?

  10. The funny part about your understandings will never grasp what Black/Dark Matter TRULY is, for one simple reason… must be Spiritual, as in your Essence, to Overstand a Spritual Entity. Black/Dark Matter is not a “what” it is a “HU.”

  11. Blacksphere’s (holes) is where all the answers are just have to gas them out with a lot of magnetism and some bubble wrap? But seriously it’s leftovers from the creation of the sphere and all the other spheres and stars. When you have stars and spheres being made you will have memory of said events and the memory is dark matter. When we look at galaxies big small they have dark matter. Dark matter is the remnants of the events. Like does a blacksphere (hole) lose the info no it’s dark matter. And it collects with mergers and other events and that’s what you see the leftovers or information that said events occurred.

  12. Also the dark info (matter) will continue to accumulate around blacksphere’s as they are the only thing that dark info (matter) that can show it’s existence in the first place that’s why there is a direct correlation to blacksphere’s (holes). Dark matter accumulates that’s why we see so much around galaxies. And magnetism holds it in place

  13. So dark matter accumulates with mergers and grows someone say prize who said that.

  14. And I also agree that the uni is much older that 13.8 billion years old sure an event may have happened 13.8 billion years ago but not the start of the uni. Maybe time started then? But not space and maybe that’s where the theories breakdown because space and time might not have started at the same time. Someone say prize?

  15. When we say spacetime maybe that’s the wrong term maybe it’s space and time. Was space here before time of course it was. I’m just rambling lol. I love this stuff lol love the comments it’s great faudder. Keep up the great works.

  16. Been thinking about a big bang theory and I say this. Before the hot big bang the hydrogen in the universe was a snow ball of hydrogen and it may have lasted for 380 000 years prior. Now if that was the case what happens to the core of that ball of hydrogen I’m thinking the core would heat up according to pressure and melt. Now would it turn to liquid or gas or both before the bang occurred. The universe doesn’t need to be hot atoll I think the opposite is probably true. I’m only a little way in thinking about that particular issue but the more I think about the problem the more it seems to be correct. Snow hydrogen? In the primordial universe it would seem was very cold and we know how easy snow sticks together and at a certain size it would form a ball?

  17. I don’t mind that you put ads on my comments as long as I get compensated for it. We can share and everybody is happy. Then we can really get deep on some of this stuff hows that sound?

  18. I thought I answered the question of dark matter? So what part of the answer is giving you trouble? Dark energy let’s see so everywhere you look in the night sky you see stars and galaxies. What are those things doing there creating energy and in doing so you have dark energy. That energy permeates the universe and is dark energy.

  19. GOD BLESS give GOD Praise. An GLORY your dark matter is THE LORD THY GOD my name is Macon text me (206) 778-4365 let’s talk GOD be blessed forevermore

  20. The concept of dark matter isn’t necessary to explain galactic rotation curves if general relativity is used and not neglected in favor of Newtonian dynamics. This isn’t the only paper I’ve seen on the topic, but is indicative of how the math works:

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