Why do we need dark matter

22 dec dark matter in space - part 1

Dark matter in space - what is it, please? And why do scientists come up with this idea?

By Sonja Ornella Schobesberger

The world - the sun, the earth, the computer or we humans - consists of atoms. And those atoms, in turn, are made up of protons and neutrons and electrons. One or the other may remember this image as part of school lessons.

Well But that's only a tiny part of the whole truth.
In the first step it turned out that the world consists of much smaller so-called elementary particles. In fact, there is a whole zoo full of different particles, such as the muon, the strange quark or the Z boson. Physicists call this zoo the standard model of particle physics.

But the question arises: are these elementary particles finite enough to be able to fully describe the physics of our universe? The answer is: no! As it currently seems, not even in the slightest ...

For about 90 years, observations that we would never have expected, on the largest possible scales in the universe, have signaled that we may have missed something on the smallest scales of the particles. But what is this something?

Without dark matter, galaxies would shred apart. Without dark matter, we would not exist.

The astronomer, the genius, the pioneer, Vera Rubin provided the first really convincing observations, which prompted her and many scientists after her to formulate the dark matter hypothesis. The problem is this:

Many theories of physics enable physicists to make predictions that are incomparably precise. For example, one of these theories is general relativity. If we start from these theories, the matter that we see in the universe in galaxies and intergalactic gas clouds is not even close to being able to explain how the universe developed into what it is today.

Without dark matter we cannot explain our own existence!

If we look at a galaxy in which the stars in it rotate around the center, then there is an equilibrium of forces in this galaxy. The gravitation, the source of which is the matter in the galaxy, pulls everything to the center and the force due to the rotation pushes everything outwards. Astronomers can now measure very precisely how fast the stars rotate around the center. The velocities measured in the outer area of ​​the galaxies are so great that many times more matter would be required in the galaxy to prevent the rotation from tearing the galaxy to pieces.

But we do not see these masses of invisible dark matter. And not only galaxies need dark matter in this sense. Not even the entire distribution and large-scale structure of visible matter in the vastness of the universe could have developed into what we observe today.

We know what it is not. But we don't know what it is.

We call dark matter dark because it does not interact electromagnetically. This means that it does not interact with photons (in other words with light) like everything else we can see.

Since it has been known that dark matter cannot consist of compact objects such as stars or galaxies, of gas or of any known elementary particles of the Standard Model, physicists worldwide have been engaged in a tough competition to see who will be the first to acquire the particles that make up dark matter , correctly predicts with an accurate theory and detects it in an experiment. But more about that in the next part ...

Cover photo: Simulated density distribution of dark matter at an age of the universe of one gig year, The Millennium Simulation Project

Post photo: Jeremy Keith, CC BY 2.0 https://creativecommons.org/licenses/by/2.0, via Wikimedia Commons