Dark matter
Dark matter (violet filaments) and ordinary matter (red/yellow regions) in a simulated cluster of galaxies.
Image: Illustris project

Unveiling the nature of dark matter

Dark matter is one of the great mysteries of the universe. It is highly abundant, yet nobody knows what it is. But now, scientific instruments have become sensitive enough that soon, researchers will be able to detect the leading dark matter candidate – that is, if it exists.
For every star, galaxy and dust cloud we can see in space, there are five times more invisible, so-called dark matter.

"We do not know what dark matter is, but without it we cannot explain how the universe evolved into what we see today. Dark matter is one of the pillars of modern cosmology”, says Riccardo Catena, researcher at the Division of Subatomic and Plasma Physics at Chalmers University of Technology.

Hints of invisible matter

As early as the 1930s, the Swiss astrophysicist Fritz Zwicky noted that galaxies in nearby galaxy clusters moved faster than could be explained by the gravity of just visible matter. He therefore suggested the existence of invisible matter. But the idea did not get much attention.

However, when the American astronomer Vera Rubin studied the rotation of galaxies in the 1970s, she discovered the same thing – the velocities of the stars were too great to be explained by visible matter alone. Now, the science community began to take the idea of dark matter seriously.

Dark matter has also been shown to be indispensable to the formation of the structure of the universe.

“In the early universe, the gravitational force, which pulls matter together, and radiation, which draws matter apart, struggled against each other. In order for galaxies and galaxy clusters to form as quickly as they did, a dark component that is not affected by radiation is needed”, explains Catena.

An unknown particle

Most of the evidence indicates that dark matter consists of some type of particles – particles that neither absorb nor emit light, or other radiation, are stable for billions of years and move at a significantly lower speed than light.

No known particle matches these criteria. Therefore, scientists are looking for a new particle. The most popular hypothesis is that it is a particle about as heavy as an atomic nucleus and which interacts weakly with common matter, a so-called weakly-interacting massive particle, or WIMP.

If the hypothesis is correct, the earth passes through clouds of WIMPs all the time. Most of them pass unaffected right through the earth, but in theory, some of them should happen to hit the nucleus of an atom in a detector. If so, there is a chance to detect it.

Weak signals to interpret

But the signals are extremely weak. One of the leading experiments, Xenon1T, is located in Italy under a mountain to shield its huge detector from disturbances such as cosmic rays.

"The experiments are becoming increasingly sensitive. If WIMPs exist, we should find them within ten years”, says Catena.

He himself is a theorist and calculates what the signature signals from WIMPs would look like, in order for those running the experiments to know what to search for.

“I also design strategies for how to interpret the measurements, so that we can learn as much as possible about the WIMPs once they are found.”

In June he will arrange a conference for both experimentalists and theorists in dark matter research. Several prestigious speakers have already accepted invitations.

“The detection of WIMPs may come at any moment in the coming years. We must be prepared to interpret a discovery with optimal strategies, in order to learn as much about them as possible”, says Riccardo Catena.

Text: Ingela Roos, ingela.roos@chalmers.se

Published: Fri 04 May 2018. Modified: Tue 15 May 2018