Is dark matter necessarily transparent

Dark matter: mysterious forces in space

The universe is shaped by two mighty powers: the physicist Laura Baudis explains what researchers know about these forces - and how complex experiments are supposed to uncover their puzzling properties

Professor Baudis, you are working on one of the great puzzles of the universe, dark matter. Why?

Prof. Laura Baudis: Dark matter is a very special form of matter: we cannot see it because it does not shine. It neither emits light - in contrast to the visible matter of a star. It still reflects light - unlike the visible matter of which stones, clouds or our bodies are made, for example. The dark matter is therefore completely transparent and completely invisible to us.
We assume that, like visible matter, it also emerged from the Big Bang and thus shaped the development of the universe from the beginning. But in contrast to visible matter, we do not know its composition. Using complicated measurements, we can draw conclusions about the mass of dark matter in the universe and how it is distributed. However, it is not yet possible to say with any certainty whether it consists of one type of particle or of different types.
And to contribute to the solution of this riddle, that excites me immensely.

Why didn't stars form from dark matter?

One of the reasons for this is that their particles apparently behave very differently than those of visible matter. In the world we know, the smallest particles - such as quarks - interact with one another in multiple ways. An example: Several quarks combine to form larger particles, the protons and neutrons. These, in turn, form the nuclei of atoms with tiny electrons buzzing around their shells. And the nuclei can fuse together under high pressure - that happens in stars. This is how other items are baked. Hydrogen becomes helium, then lithium, later carbon, oxygen and iron. And various elements can ultimately form compounds - hydrogen and oxygen, for example, form water.
The particles of dark matter, on the other hand, hardly interact with one another; they probably do not form any more complex structures even at the particle level. They tend to buzz around individually and freely, like the particles in an ideal gas.

What else is known about the mysterious particles of dark matter?

Amazingly little. Although they make up more than 80 percent of the matter in the universe and there are numerous projects that aim to provide evidence, it has not yet been possible to detect even a single particle of dark matter directly with the aid of a detector.
For a while, the researchers thought that dark matter could actually be neutrinos, tiny elementary particles that travel at almost the speed of light and also rush through ordinary matter. We now know, however, that the total mass of the neutrinos is not sufficient to explain the effects of dark matter.
So there must still be completely unknown particles behind it.

Where does dark matter occur?

It exists everywhere in the universe, it pervades the entire universe. Dark matter is also present here on earth; it even flows through us constantly: the particles that make up dark matter simply blow through us. We just don't notice anything about it.

Why don't we notice that the particles are flowing through us?

Because they do not - or hardly - interact with the atoms that make up our body. Therefore we do not feel it, we have no sensors to perceive it. But dark matter is always there.

How many of these particles are flowing through us?

According to a common theory, an average of 100,000 dark matter particles rush through an area the size of a thumbnail per second. That may sound like a lot, but it is actually comparatively little. Because the density of dark matter is here on earth, where the visible matter is extremely compact, around 20 orders of magnitude below that of air. Therefore, their mass does not exert any special attraction on our body. Their effect is only noticeable on a larger scale - on the level of galaxies.

Dark matter seems to elude the known world. How did physicists discover its existence?

As early as the 1930s, the Swiss astrophysicist Fritz Zwicky postulated the existence of an invisible form of matter. The scientist had observed the speed of several distant galaxies that form a kind of cluster. On the basis of complex calculations, Zwicky determined that the galaxies should actually fly apart. If there were only visible matter, i.e. mainly the mass of the stars and the gas in between, galaxy clusters should not form at all - their gravitational force would simply not be large enough to bind the star archipelagos to one another.

What did Zwicky accept?

There had to be far more than the visible matter - something that holds the galaxies together like a kind of glue. But Zwicky's theory was ridiculed by colleagues and his discovery was ignored for a long time.
It was not until the 1970s that the US physicist Vera Rubin made another revolutionary observation that was ultimately taken seriously by astrophysicists around the world: She determined the speed at which stars move around the center of a galaxy. The researcher found that the different speeds of the stars can only be explained by the existence of a large, unknown mass. If only visible matter were present, then, according to the laws of physics, those stars that are far from the center of a galaxy would have to move much more slowly than they actually do.

Was there any other evidence of the existence of dark matter?

Yes, for example the gravitational lens effect: Dark matter surrounding galaxies deflects the light further away from celestial bodies lying far behind them due to their immense mass and the resulting gravitational force. The result: When viewed from Earth, the objects behind the galaxy appear distorted - just as if they were being viewed through a kind of lens.

And there is another reference to the existence of dark matter. There is a radiation that fills the whole universe, it comes from the early days of the universe and is a kind of echo of the big bang: the cosmic background radiation. By precisely measuring them, it is possible to calculate how much mass was already there in the primeval times of the universe.

These physical quantifications reveal that the mass is much higher than that of which the world we perceive - such as stars, planets, asteroids, gas clouds - consists. One could also say: one has weighed the universe and found that a considerable part of matter is still missing.

How much dark matter is there?

Dark matter makes up about 85 percent of total matter in the cosmos, the rest of which is visible.

How did dark matter influence the development of our universe?

In a sense, it played the main role in the development of the universe, because it gave structure to the cosmos. None of the spectacular, gigantic structures that we see in the universe - such as galaxies, galaxy clusters or network-like associations of thousands of galaxy clusters - can be explained without the power of dark matter.
In the meantime, physicists can use modern computer simulations to understand the structure formation and in particular the formation of the galaxies in the universe very precisely. These simulations cannot do without dark matter.

What if you leave out dark matter in simulations?

If researchers only take into account the particles that make up visible matter in their calculations, neither stars nor galaxies are formed. Without the dark matter, space would look rather dreary today: There would be atoms - hydrogen, helium. But the cosmos would be dark, no sun would shine, no planets buzzing around. The universe would be filled with a thin gas, mainly hydrogen.

How did dark matter bring structure into space?

In the early days of space, dark matter was not evenly distributed in space. In some regions there was a little more, there the dark matter particles were buzzing around a little closer. Since the particles have a mass and therefore develop the force of gravity, over time - driven by gravity - more and more dark matter collected in the dense areas. Ultimately, huge spherical accumulations of dark matter, the halos, formed.

These spheres developed a tremendous gravitational force, which also attracted huge amounts of visible matter. The halos so to speak sucked in hydrogen atoms from the environment, which over time formed gigantic clouds.

And only when the hydrogen was brewing like this could it develop a gravitational force itself that was relevant for the further development of the universe. As a result, so much hydrogen gas accumulated in some parts of the cloud that atomic fusion occurred - and the first stars were ignited.

In the midst of the halos of dark matter, gravity was strongest, this is where most of the hydrogen gas collected, and this is where most of the stars ignited. And the luminous celestial bodies finally formed - also directed by the tremendous power of dark matter - into those glittering star islands that we can marvel at billions of times in space: the galaxies.

You can read the full interview in GEO Kompakt No. 51 "The Birth of the Universe".