How do we find the invisible?

Most of the matter in the universe is dark matter, invisible to the human eye. The long-awaited discovery of the dark matter particle would open the road towards a completely new theory of particle physics.

The astronomer Fritz Zwicky was the first person to suggest the existence of dark matter in the 1930s. He studied the movements of galaxies and found that the mass of stars alone would not be sufficient to keep the galaxies together. Vera Rubin and Kent Ford Jr came to similar conclusions in the 1970s. The Big Bang theory gave scientists the opportunity to calculate the number of protons and neutrons in the universe from the relative amounts of hydrogen and helium. 

It turned out that if the universe only contained the kind of matter we know, i.e., protons, neutrons and electrons, it would appear very different than it does. This means that most matter in the universe is something we cannot see.

Even though we have been aware of the existence of dark matter for decades, we still don’t know what it is. We know that there is about five times as much dark matter in the universe as there is other types of matter, and we know that it is not electrically charged. We know that it does not involve strong nuclear energy. If it did, we would have found exotic atoms or nuclei with dark matter.

The search for dark matter entered a new era in the 2010s when the Large Hadron Collider (LHC) was engaged in CERN. In many models, dark matter interacts with normal matter through weak nuclear force. The LHC can locate the dark matter particle involved in such weak interaction if it is not too heavy.

We may solve the mystery of dark matter during the coming years. If we do not find the dark matter particle, many of the current dark matter models will be tossed in the trash. But if such a particle is found, scientists will find themselves on the road towards a new kind of theory of particle physics.

We may solve the mystery of dark matter during the coming years.

The current understanding of particle physics has been compiled into a theory known as the standard model. The standard model of particle physics includes all known particles and all known forces, with the exception of gravity.

Cosmology has its own standard model, which is based on the properties of the cosmic microwave background. 

The cosmic microwave background was created when the universe turned from opaque plasma into transparent gas. The light emitted by the plasma is still measurable as microwaves. The variations in the density of the early universe – the primordial fluctuations which are the seeds of galaxies – are apparent as minute changes in temperature in the microwave background. 

The problem is that the standard model of particle physics is not compatible with the one of cosmology. The properties of the microwave background, the accelerating expansion of the universe and the movement of the stars in the galaxies require the existence of dark matter and dark energy. Dark matter must exist for galaxies and smaller cosmic structures to have formed as rapidly as they did. When researchers have simulated the development of the universe, they have found that dark matter accelerates the generation of primordial fluctuations. These fluctuations lead to the creation of stars and galaxies.

Without dark matter, life as we know it would not exist! The standard model of particle physics does not recognise dark matter or explain dark energy.

Most scientists assume that there is at least one more fundamental particle in the universe in addition to the protons, neutrons, electrons and neutrinos we know –the dark matter particle. Discovering this particle is one of the biggest challenges in particle physics.

Without dark matter, life as we know it would not exist!

Particle physics uses detectors to observe particles. In most systems, the particles ionise matter inside the detector, and the resulting electrical current can be measured. Or they collide with atoms in the detector, turning their kinetic energy into heat, which can then be measured. 

Dark matter leaves no trace in traditional particle detectors. This means that researchers are essentially looking for the invisible.

But how can they make the invisible visible?

Scientists look for dark matter by colliding protons and other particles in particle accelerators. Dark matter in itself cannot be seen, so researchers have to look for reactions which result in something visible flying off in the opposite direction in a collision. If the detectors show particles flying in one direction and nothing going in the opposite direction, the particles may be colliding with dark matter. These experiments use the available particle ratios of known particles and look for clear anomalies from the predictions.

Another way to look for dark matter is to construct a large detector underground, the most sensitive of which is the XENON1T experiment in Gran Sasso, Italy, comprising a tank with more than a ton of liquid xenon ( Such detectors can find situations in which a dark matter particle collides with an atom in the detector and manages to change the atom’s energy state. This generates visible light. Collisions like this are rare. 

Similar collisions can also occur for other reasons, but it is possible to discern collisions caused by dark matter from others. 

If you run around in a circle in the rain, you will get more drops on your face while running against the wind than when you run with the wind at your back. The Earth goes around the Sun, and in terms of dark matter, the Earth has a headwind in June and a tailwind in December. If the number of collisions fluctuated accordingly, it would be a signal of the presence of dark matter. However, no such annually repeating periods have been observed in experiments. 

This generates visible light.

If the dark matter particle is found before the next new particle, it will open the gates to solving the other problems in the standard model of particle physics. The problems relate to the fact that we do not understand why the standard model is the way that it is. When nineteen parameters are fed into the model along with known particles and known forces, it works well. However, we do not know why the model requires these specific parameters. 

Particle physicists are hoping that a more comprehensive theory would explain the seemingly arbitrary features of the model. Some of the problems in the standard model can be resolved through supersymmetry or additional dimensions. If dark matter is discovered, its properties will help us choose the best way to expand the standard model. If it is not discovered in the next few years, we can abandon certain options.

So far, no signal of dark matter has been found in LHC experiments or anywhere else, but scientists continue their quest for the invisible. If they succeed, we will be able to understand a larger area of the universe.

Harri Waltari

Postdoctoral researcher, particle physics phenomenology
University of Helsinki, Department of Physics

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From the Researcher’s Pen is a series of articles in which researchers describe their work. This article was produced by Uutistamo in cooperation with the University of Helsinki.