What is Dark Matter Made Of? Leading Theories Explained: Axion, Wimp, Machos

What is Dark Matter Made Of? Leading Theories Explained: Axion, Wimp, Machos

What is dark matter made of? Leading theories explained: Axion, Wimp, Machos

When you look at the night sky what do you see? You see a few stars, but you mostly see dark. What is in that dark? Is it all a vacuum, like we've been taught in school? 

What you may not realize is that the night sky really would look like this, if you could see all the hidden matter that's invisible to our eyes. 

One of the biggest, if not the biggest question in science right now is, "what is the universe made of?" 

Sure, I've talked about the standard model and quantum field theory in several articles, which try to explain what the universe is made of. 

But these, so far, only attempt to explain what the visible matter in the universe is made of. It turns out that visible matter - that's everything that you can see on earth, and in space, with the most powerful telescopes, is only a minuscule portion of all that exists. 

There's about six times more matter in the universe than what we can see. This invisible matter is called "dark matter." 

This means that what we're seeing are only the waves on top of the ocean. We're not really seeing the ocean itself. Yet, without this ocean, we would likely not exist.

So If This Dark Matter Is Invisible, How The Heck Do We Know That It's Even There? And If It Exists, Then What Could It Actually Be?

You'll be surprised how much we know about it. Let's look at what our best theories in science have to say about it,

What is dark matter made of? Leading theories explained: Axion, Wimp, Machos

It turns out that we can detect invisible matter by its effect on the visible matter that we can see and detect. The first evidence of dark matter came from Swiss astronomer Fritz Zwicky in the 1930s, when he saw that the galaxies in clusters, like the Coma Cluster, were moving at very high speed. In fact, they were moving so fast that the galaxies could not stay gravitationally bound, and should have gotten ripped apart, unless they had, he figured, about 100 times the matter that he could see visibly. He coined the term "dark matter" to describe this invisible mass. The problem was nobody believed him. He was kind of an eccentric guy, and not very likable. Most scientists either ignored him, or didn't believe him.

What is dark matter made of? Leading theories explained: Axion, Wimp, Machos

It wasn't until almost 40 years later, in the 1970s, that observations by Vera Rubin confirmed the idea of dark matter. She measured that the velocity of stars on the outer edge of galaxies was about the same as those closer to the center. But the mass at the edge of galaxies, based on visible stars, gets smaller. This appeared to be impossible to explain without the concept of unseen matter causing gravitational effects on the outer edges of galaxies. 

What is dark matter made of? Leading theories explained: Axion, Wimp, Machos

If you compare the orbital speed of planets around the Sun, you'll find that the speed slows down the further away from the Sun you go. The curve of speed versus distance from the Sun follows a one over the square root of distance relationship. 

What is dark matter made of? Leading theories explained: Axion, Wimp, Machos

However, when you look at the orbital speed of stars in the Milky Way galaxy, you find that almost all stars are orbiting at the same speed - about 200 kilometers per second. And most other galaxies seem to show the same general trend. 

What is dark matter made of? Leading theories explained: Axion, Wimp, Machos

Near the center of the galaxy there's enough visible matter to account for the speed. But as you get further out, there's a greater and greater mismatch between the orbital speed of stars and the visible matter. This is the reason it is thought that most of the dark matter forms a kind of halo around the galaxy. There is likely less of it near the center of the galaxy, but a lot more the further out you go. Most other galaxies seem to show the same general trend.

What is dark matter made of? Leading theories explained: Axion, Wimp, Machos

Now, how do we know the dark matter is not just ordinary matter that is hard to see - like dim stars, or black holes? This was thought to be a possibility decades ago, and some of these were categorized as MACHOs or, Massive Compact Halo Objects. But today, we have so much observational data that this possibility has been all but ruled out. The distribution of dark matter, based on the rotational curves, gives us a clue. It appears to be distributed evenly, as well as like a halo around the outer edges of galaxies, and not clumped up like visible matter is.

What is dark matter made of? Leading theories explained: Axion, Wimp, Machos

This can be explained if we theorize that this matter interacts very weakly with itself. But ordinary matter does not interact weakly and thus tends to clump together. Ordinary matter would likely not be so evenly distributed. And research as recent as 2019, from a team in Japan, appears to have ruled out primordialblack holes as being the source of dark matter. In addition, there are other more technical reasons, having to do with the rate at which new elements were formed in the early universe, which indicate that only about 5% of the energy density of the universe can be explained by ordinary matter. The rest has to be dark matter and dark energy.

I'll be getting into details of dark energy in a future articles. By the way, most of the ordinary matter in galaxies is not in the form of stars and planets, but interstellar gases. If you added up all the mass of this gas, it would be about 3 times as much as the mass of all the stars and planets of the universe put together. So a pie chart of the mass of the universe looks like this - 4 percent stars and planets, 11 percent gas, and 85 percent dark matter.

What is dark matter made of? Leading theories explained: Axion, WIMP, MACHOs

The bullet cluster, is I think, the most interesting evidence showing how the behavior of dark matter is not like ordinary matter. This is an enhanced photograph of two clusters of galaxies passing through each other. 

What is dark matter made of? Leading theories explained: Axion, WIMP, MACHOs

In a computer simulation of this, what you can clearly see is that the gases of two galaxies are interacting with each other and heating up, to emit x-rays, which are visible as bright pink areas. This is the ordinary matter of the two clusters. The majority of the matter, however, as indicated by the gravitational lensing, is in the blue areas of the photo. This is the dark matter that simply did not interact with the ordinary matter, or with itself, as the two clusters collided, and thus, passed right through.

So you can clearly see that most of the mass is not where most of the ordinary matter is. There are some physicists who argue that dark matter doesn't really exist, and the rotation curves can be explained if the laws of gravity were different at larger distances. Again, this behavior in the bullet cluster would be hard to explain if that's the case. Inventing a new particle is probably less crazy than proving the laws of gravity wrong, given the mountains of evidence that prove Einstein's general relativity is correct.

So If It Is Not Dim Ordinary Matter, And Not Due To Changes In Gravity, Then What The Heck Is It?

Let's look at what properties the potential Dark Matter particle would need to have.

  1. First It obviously must be dark. This means it does not emit any light and doesn't interact with anything to emit light, or any kind of electromagnetic radiation.
  2. Two, it must of course interact through gravity, so it cannot be a zero rest mass particle, like photons.
  3. Three, it must not interact with itself, or interact very weakly with itself.
  4. Four, it must be cold. That is, it is not moving at very high speeds, because if it was, then after the Big Bang, it would have just kept on going, and not slowed down to form halos around ordinary matter.
  5. And five, it must be stable. That is, it does not decay, because if it decayed into other particles, it wouldn't still be here now in such large quantities, 13.8 billion years after the Big Bang.

if it's a particle, could it be hiding in plain sight in the standard model of particle physics? We need a particle that does not decay and is neutral. The only massive neutral particle in the standard model that is stable is the neutrino. But because neutrinos are so light, they're not cold. They move very fast, so they're like hot dark matter. This would preclude neutrinos from making the large structures in the universe, because the neutrinos would want to move away while the ordinary matter wants to come together. So the net effect would have been a smoothing out of ordinary matter.

What If We Invent A New Particle

What is dark matter made of? Leading theories explained: Axion, WIMP, MACHOs

If we invent a new particle that does not interact with electromagnetism has no strong force nuclear interactions but does interact through the weak nuclear force, and through gravity, then we have just invented the dark matter particle. Scientists have a name for this particle it's WIMP, or Weakly Interacting Massive Particle.

Why Would This Particle Need To Interact Through The Weak Nuclear Force?

because it turns out that if we ask the question, "what is the rate of interaction of dark matter particle and antiparticle annihilation that you would need to have to give us the correct ratio of dark matter that we observe today?" the answer is that they would need to interact with the strength EXACTLY equal to the strength of the weak nuclear force.

This coincidence is called the "wimp miracle." And it so happens that there is a model in particle physics that predicts exactly such a particle. This model is called supersymmetry. Scientists like the WIMP because it was not invented for the purpose of solving the dark matter problem, but comes about naturally in string theory. And string theory requires supersymmetry to work.

So What Is Supersymmetry?

supersymmetry is a special kind of symmetry between force particles, the bosons, and matter particles the fermions. Supersymmetry says that for every boson particle, there would be a corresponding fermionic particle, and vice-versa. And it would have the same charge, strong and weak nuclear force interactions. It would have a different spin, and because of "symmetry breaking" it could have a much higher mass.
Now, supersymmetry has not been found to be confirmed in nature. But if supersymmetry is true, then the Dark Matter mystery could be solved, because there would be a perfect candidate for a Dark Matter particle in supersymmetry. And it is called the Neutralino. The Neutralino would be the lightest supersymmetric particle of the photon, the z boson, and the Higgs bosons. So supersymmetry would double the particles in the current standard model.

Is This Complication Worth It?

Well, Dark Matter is a benefit, but the fact that it makes string theory work is also a benefit. And it explains some mysteries as well, such as why the Higgs boson is lighter than it should be. But the biggest problem with the neutralino theory, is that we should have been able to detect it at the Large Hadron Collider. And so far, we have not. So this puts a big damper on this theory.

What is dark matter made of? Leading theories explained: Axion, WIMP, MACHOs

The Axion is the second viable candidate to be the Dark Matter particle. The Axion was invented to solve a problem having to do with something called the charge conjugation and parity symmetry problem with the strong nuclear interaction. This is also called the strong CP problem.

What is dark matter made of? Leading theories explained: Axion, WIMP, MACHOs

CP symmetry basically means that the laws of physics should be the same if a particle is replaced by its antiparticle - C symmetry, and its spatial coordinates are inverted to its mirror image - that's the P symmetry. This is a technically complicated issue for the strong force, but we can simplify it by calling it, "the electric spin problem of neutrons." It so happens that the theorized existence of a particle called the Axion solves this problem. Essentially, the problem can be more simply shown in the following way: as you know, protons and neutrons make up the nucleus of atoms. The neutron has a property called a magnetic moment, or spin.

What is dark matter made of? Leading theories explained: Axion, WIMP, MACHOs

This means that if you put the neutron in a magnetic field, the neutron will have a spin. The reason for this spin, even though the neutron has no charge, is because the neutron is made of three quarks that have spin. And all of these combine to give the neutron a spin. And this is confirmed by observation.

What is dark matter made of? Leading theories explained: Axion, WIMP, MACHOs

But the neutron, according to theory, should also spin if exposed to an electric field. However, for some reason, it does not. No one knows why. And this is the problem. The lack of spin, it turns out, is dependent on one of the fundamental constants of nature, called "theta." 

This theta has to be zero, or close to zero for this observation of no spin of neutrons to happen. This is one of the constants like the gravitational constant, or the Planck's constant.

What is dark matter made of? Leading theories explained: Axion, WIMP, MACHOs

But theta should vary anywhere from negative pi to pi. There's no reason that it should just happen to be zero. This seems to be a huge fine-tuning coincidence! And physicists don't like coincidences. 

What is dark matter made of? Leading theories explained: Axion, WIMP, MACHOs

In 1977 two physicists Roberto Peccei, and Helen Quinn proposed a theory which solved this problem, by creating a new field for theta instead of it being just a fundamental constant. And since most fields seek to maintain the lowest possible energy state of zero, that's what theta would also seek to be - zero.

But if theta is a field, like the electromagnetic field, or the quark field, it means that it will have a particle associated with an excitation of the field. This particle of the theta field is called the Axion. But due to the nature of the theta field, the Axion is predicted to have an extremely low mass. There are a couple of reasons why Axions make an attractive Dark Matter particle candidate.

First, they do not require the existence of supersymmetry. And they can be placed in the standard model as a much lighter cousin of the higgs boson. But it's very low mass would be made up for in very large numbers. If you had a cubic centimeter of space, it would contain roughly one WIMP particle, if WIMPs were dark matter. But if Axions were dark matter, one cubic centimeter of space would contain ten quadrillion, or roughly 1016 Axions.

Axion experiments are being done in Hamburg, Germany and in CERN. But so far, nothing has been detected. So while scientists have placed big bets that dark matter is likely the WIMP and/or the Axion, no proof, or smoking gun evidence has been found.

But this is not atypical of a story in science. Things this important can take decades to resolve. It is possible that we just don't have the right tools currently to find the particles this elusive. And we might just have to be patient. Or, maybe, all this is wrong, and we need a completely new theory to explain dark matter. Either way, I think this is a really exciting area of research. And it's going to keep current and future physicists busy for a really long time.


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