Can we drop the dark matter hypothesis

Dark matter: What else is out there?

What could dark matter be made of? Why is it so hard to find? The Heidelberg physicist Susanne Westhoff, who is looking for her at a particle accelerator herself, takes us on her search in the vastness of darkness. If the discovery succeeds, we will not only better understand a large part of the universe, but also its history.

What is dark matter?

Matter, no question about it, that's all we see, what we can touch, what we're made of. If you look closer, it consists of atoms, i.e. compact atomic nuclei surrounded by tiny elementary particles, the electrons. Because electrons send and receive light, matter is visible to us.

So everything we see is matter. But the reverse is not true: there is matter that we cannot see and otherwise cannot perceive with our senses. In the entire universe there is even five times as much invisible matter as there is visible matter. The Swiss astronomer Fritz Zwicky was the first to notice this. As early as the 1930s he postulated this invisible type of matter and named it "dark matter".

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Has anyone seen them yet?

Of course, Fritz Zwicky could not see dark matter either, but he could deduce it from his astronomical observations. He analyzed the movements of star nebulae in the Coma Cluster, a huge galaxy cluster about 300 million light years away from the Milky Way, our home galaxy. The trick was that Zwicky could predict these movements using the law of gravity. He knew the mass of the Coma cluster, the gravity of which was supposed to direct the nebula along certain paths. The researcher was all the more astonished when his observations did not seem to match his prediction at all. To explain the star movements, the Coma cluster had to be 400 times heavier than Zwicky had calculated. It couldn't be because he might have missed a few galaxies. There had to be an enormous amount of heavy matter that he couldn't see with the telescope. Enter: Dark Matter!

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Then how do you know they exist?

What has been questioned again and again since Zwicky's discovery is now a certainty: it really does exist, dark matter. To prove this, astrophysicists have developed sophisticated methods with which they can detect dark matter in our galaxy and in its neighborhood. In the meantime, the various measurements are so precise that dark matter cannot simply be discussed away as a possible measurement error. So the evidence that dark matter exists is so strong that it has become an integral part of our theory of the universe.

One of the most impressive techniques is the gravitational lens: similar to how a glass lens bundles light into a narrow beam, a galaxy cluster can also bundle light. According to Albert Einstein's theory of relativity, the gravitational field of the galaxies bends space-time, so that rays of light from star clusters in its vicinity no longer fly straight ahead, but instead reach our telescopes on curved paths. The more dark matter is accumulated, the more it bends the light paths. So what may seem like science fiction is a prism made of dark matter.

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What would a universe look like without them?

If it weren't for dark matter, our universe would look pretty boring. Their gravity holds the galaxies together, including our Milky Way. Without dark matter, the galaxies would not even have formed and the universe would have remained a cloudy soup after the Big Bang. And we humans, the most extreme and unlikely accumulation of matter, would not exist either.

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What could dark matter be made of?

As certain as the researchers are that there is dark matter, opinions differ as to what it could consist of. We can detect dark matter in the universe, but microscopically it is a great mystery to us. In any case, dark matter has to be electrically neutral, otherwise it would emit light like electrons and could be seen with the telescope.

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There are currently two much discussed hypotheses for the nature of dark matter: The first hypothesis is that it consists of black holes. Black holes are places in spacetime where gravity is so strong that light and everything else can get in, but hardly comes out. So perfectly dark places that actually exist in the universe. As dark matter, however, they would have to have formed shortly after the Big Bang in order to help the galaxies. How exactly this should have happened is not yet clear.

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Endless expanses, this way

The second, equally spectacular hypothesis interprets dark matter as a new species of elementary particles that interact with visible matter through a very weak force. For a long time, neutrinos were hot candidates for dark matter particles. Neutrinos are the lightest elementary particles we know. Every second, tons of it are generated in space, which then hit us here on earth. But we don't notice anything because neutrinos interact so weakly that they penetrate us virtually unaffected. Just like dark matter! In the meantime, however, we know that at most a very small part of dark matter could consist of neutrinos. They are just too light and fleeting to explain the structure of galaxies.

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Which candidates are the best?

Good candidates for dark matter must withstand all measurements in space and all searches in the laboratory, so they should be consistent with the data. Black holes, which we already know from cosmology, would be an elegant solution for which we would not have to postulate any new particles. However, the hypothesis is severely limited by cosmological investigations, for example with the gravitational lens: If dark matter actually consists of black holes, these would have to have a very specific size according to the principle of exclusion. But why? And how did these black holes come about in the early universe? These are open questions in current research.

On the other hand, it seems plausible that dark matter could be new particles, since we can describe all visible matter in terms of particles. There are plenty of good candidates out there, and each one has its own cosmological story. This abundance may be one reason why we have not yet found a dark matter particle in the experiment. If the question of the best candidate could be answered precisely, we could search for it specifically.

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How many particles of it pass through us per minute?

We only know the density of dark matter. In our Milky Way, that's about one gram per billion cubic kilometers *, about as much as a grain of sand in the Mediterranean. We are surrounded by this thin haze, which moves through space at an average of 220 kilometers per second. Dark matter can do in a second what an ICE does in an hour. How many particles pass through us per minute depends on how heavy they are. Suppose a dark matter particle weighs as much as a gold atom, then about a billion particles passed through us every minute.

* Correction: At this point "cubic meters" was incorrectly mentioned.

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Can such particles cause cancer?

Dark matter particles, if they are actually new particles, hit us all the time. However, their interaction with our body is so weak that the particles simply fly through us and we do not feel any of them. We also don't need to worry that dark matter will make us sick. And even if the few collisions had an effect: Dark matter has been around so much longer than we have that we have long got used to it. If the particle collisions affected our health, we would never have evolved that far.

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How could we find dark matter?

It seems paradoxical: we are looking for something that we know has to be there. But there are so many ways it might look that we don't know where to start looking. The best thing we can do is make a hot candidate and hunt him down until he gives up. Translated into physicist German, this means: We use a particle accelerator, for example the large LHC at the European Research Center Cern in Geneva, and try to convert visible matter into dark matter there.

With this small intervention in the universe, we learn a lot about how dark matter particles communicate with visible particles. Anyone who can prove the artificially produced dark matter in this way receives the Nobel Prize and solves one of the greatest puzzles of our time. So far, however, no one has succeeded, perhaps because dark matter only interacts very weakly with visible matter. So the challenge is to construct an experiment that can measure extremely weak forces and thus directly detect dark matter as particles.

For a few years now, the search among particle physicists has been booming so much that new experiments are actually being built that can measure such minimal forces. One of them is fiber, a satellite of the LHC, which is currently screwed together deep underground in a supply tunnel of the accelerator ring. Fiber can detect particles that are generated at the LHC and live a very long time before they disintegrate in the detector. Are they perhaps the mediators of a weak interaction between visible and dark matter?

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Are there people who want you to be found?

Researchers have installed the Dama experiment deep under the Italian Gran Sasso mountain range, in a branch of the motorway tunnel. There they wait for dark matter particles from space to hit the detector. After years of measuring, the sensation: the scientists actually saw a signal