September 11, 2023 by glandini

Nowadays, 2,500 years after Democritus, we know that, to some extent, he was right: matter is made of atoms, extremely small particles that combine to form complicated structures such as a stone or a human being. However, Democritus was not completely right: atoms are neither unbreakable nor elementary, as they are composed of even smaller particles. Every atom has a nucleus, made of protons (particles with positive electric charge) and neutrons (particles with no electric charge) bound together and a certain number of electrons (particles with negative electric charge) that orbit around the nucleus. The number of electrons (which is equal to the number of protons, as the atom must be electrically neutral to be stable!) determines the type of element and the chemical properties of the corresponding material (hydrogen, helium, iron and all the elements of the periodic table).

Pictorial representation of an atom.

Thanks to the great advances in both theoretical and experimental physics in the 20th century, now we know that even the constituents of the nucleus, namely protons and neutrons, are not elementary, but are made of smaller particles known as quarks. There are 6 types of quarks, referred to as different flavours, with different masses and electric charges. Only the 2 lighter ones, the so-called up and down quarks, are the constituents of protons and neutrons. 

Quarks composition of the proton and the neutron.

The heaviest quarks are not present in the matter that we observe daily. However, they can be produced in very energetic astrophysical processes or large collider experiments, such as the Large Hadron Collider (LHC) at CERN. Here, protons are accelerated with strong electromagnetic fields to reach very high energy (they almost reach the speed of light, the maximum velocity that a body can reach in nature!). Subsequently, these extremely energetic particles collide, and their collisions can create heavy particles. The most astonishing thing about collider experiments is that they can reproduce the properties of the Universe when it was extremely young, a few seconds after the Big Bang, when the history of the Universe started, approximately 13.7 billion years ago. This allows us to unify particle physics with Cosmology, the study of the birth and evolution of the Universe. The early Universe was an extremely dense and hot environment where all the particles could propagate and interact among themselves. Thanks to our experiments, we can deepen our understanding of this fascinating topic. 

So, we decomposed the nucleus into quarks. What about the electrons?  At the current state of knowledge, the electron is considered to be an elementary particle with no internal structure. Experiments in the atmosphere of the planet and at colliders showed the existence of two additional particles with the same properties as the electron but much heavier than it, known as the muon and the tau particle. They can somehow be considered as the heaviest brothers of the electron. Similarly, to the heaviest quarks, they do not compose atoms but can be easily created in the laboratory or detected in the atmosphere (they are produced through the scattering of highly energetic particles coming from Space with the atoms of the atmosphere). Electrons, muons, and tau particles are known as leptons, to distinguish them from quarks. The family of leptons is completed by 3 different kinds of neutrinos, extremely light particles with no electric charge.

So, until now, we have 6 quarks and 6 leptons, 3 of which -the up quark, the down quark, and the electron- make up all the atoms. The question that is natural to ask is: How do these particles interact? How can they be bound inside atoms? The answer is simple: there are other kinds of particles that are responsible for the interactions! These are known as mediator particles. The most famous one is the photon, which is nothing but the elementary particle of which light is made! The photon has no mass, and it is responsible for electromagnetic interactions. In other words, every time a particle with a positive electric charge and a particle with a negative electric charge meet, they exchange a photon, and as a result, the two particles are attracted one towards the other. This explains how the nucleus (with positive electric charge) and the electrons (with negative charge) are bound inside atoms! In addition to photons, we have gluons, which are the mediators of the strong nuclear interaction, which is responsible for keeping the protons and neutrons bound inside the atomic nucleus, and the W and Z bosons, the mediators of the weak nuclear interaction, which is at the base of many physical processes such as radioactivity. Finally, the picture was completed by the discovery of the Higgs boson in 2012 at the LHC. This particle has a crucial role in nature, being associated with the origin of the mass of all the elementary particles.

The theory describing the elementary particles and the fundamental interactions of nature is known as the Standard Model (SM) of particle physics.  There is a fourth fundamental interaction, gravity, that is still not completely understood in the context of the Standard Model. Among the four fundamental interactions, gravity is by far the weakest one. Indeed, it dominates at astrophysical scales (where the effect of other interactions is screened by the large distances or the huge number of particles), but it is irrelevant at the level of particle physics. Gravity is described by the theory of General Relativity, elaborated by Einstein in 1915. The unification of Relativity with the Standard Model is one of the main challenges of modern physics.

The dark component of the Universe

The Standard Model has successfully explained almost all experimental results and precisely predicted a wide variety of phenomena. For these reasons, it is considered one of the greatest achievements in science. With some abuse of notation, we can call ordinary matter whatever is made of Standard Model particles. In the context of astrophysics and cosmology, it is also referred to as visible matter, since, thanks to its interactions with photons, it can emit and absorb light. The emission of light is, for example, a crucial ingredient in the evolution of a star and the reason why we can observe it with telescopes. 

Up to this point, one could think that the Standard Model can explain everything in nature and that all the Universe is made of visible matter. However, the picture turns out to be more complicated than expected. Despite its great successes, there are some phenomena that the Standard Model is not able to explain. Astrophysical and cosmological observations, which I will discuss in the following, suggest that visible matter is not the only form of matter present in the Universe. On the contrary, an additional form of matter, somehow invisible to us and our instruments, is required to explain the experimental data.

The first claim was presented by the Swiss astronomer Fritz Zwicky in 1933, while he was investigating clusters of galaxies. A cluster of galaxies is an astronomical structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity. Zwicky’s study focused on the measurement of the velocity distribution of the galaxies in a particular cluster, the Coma cluster. The astonishing conclusion he reached, applying standard gravitational physics equations, was that the only way to explain the observations was to assume that the total amount of matter present in the cluster was larger than the visible one. In other words, if one only considers the amount of matter in the cluster that can be seen with telescopes, it is impossible to explain the results for the velocity distribution. A new kind of invisible matter should be present inside the cluster. The reason for its being invisible comes from the fact that it is unable to interact with photons (or its interactions are extremely weak), and therefore it cannot emit light. No light means no observation by the telescopes. However, this invisible matter interacts gravitationally, modifying the theoretical predictions for the velocity distribution inside the cluster. Physicists started to refer to this exotic and light-phobic form of matter as Dark Matter.

Today, 90 years after Zwicky publications, the evidence for the existence of Dark Matter is overwhelming. The analysis performed by Zwicky has been extended to several clusters of galaxies as well as single galaxies, with analogous results. Additional evidence comes from the astronomical system known as the Bullet Cluster. This consists of two colliding clusters of galaxies. A detailed analysis of the structure of this system showed that the only way to explain the dynamics of the collision was to assume the presence of extra invisible matter in addition to the visible one, in perfect agreement with the Dark Matter claim. Finally, nowadays, the strongest evidence for Dark Matter comes from cosmology. As I mentioned before, the Universe started with the Big Bang. In its first epoch, it was extremely dense and hot, and all the particles propagated at the speed of light, continuously interacting among themselves. About 400,000 years after the Big Bang, the photons that were present in the Universe stopped interacting with other particles and started propagating freely. These same photons can be observed today with radio telescopes, where they manifest as a faint and almost uniform background of electromagnetic signals known as the Cosmic Microwave Background or CMB. The CMB photons keep information on the properties of the Universe at the moment in which they started their free propagation. The corresponding signal can be interpreted as a picture of our Universe at those early times, the first-ever image of the Universe!

The Cosmic Microwave Background: a picture of the Universe, 400,000 years after the Big Bang.

Thanks to the CMB measurements, we can determine the precise composition of our Universe. Indeed, the CMB signal allows us to distinguish the effects of visible matter from Dark Matter. The implications are quite surprising: to explain the evolution of the Universe we must assume that, at present, Dark Matter is much more abundant than visible matter (more precisely, about five times more abundant). In other words, not only Dark Matter exists, but most of the matter in the Universe is dark and not made by ordinary atoms! This may look counterintuitive: everything we experience in our lives is made by ordinary, visible matter, while we cannot perceive at all the presence of Dark Matter. So, how is it possible that such a huge amount of matter can escape our detection? The answer to this question lies in the properties of Dark Matter. As explained before, Dark Matter is invisible, because it is unable to emit or absorb light. Therefore, we cannot see it, neither with our eyes nor with the most sophisticated telescopes. Furthermore, Dark Matter is unable to communicate with the other mediator particles of the Standard Model, so it cannot participate in particle physics processes driven by strong or weak nuclear interactions. Consequently, the only way through which Dark Matter can communicate with us is through gravitational interactions. This matches our expectations: we can “perceive” the presence of Dark Matter in astrophysical systems, where gravity dominates, but we cannot easily detect it in particle physics experiments, where gravitational interactions are irrelevant.

For completeness, I will briefly mention that Dark Matter is not the only big mystery permeating our Universe. Another set of astrophysical and cosmological observations concluded that the greatest part of the Universe is not made by either Dark Matter or visible matter but by a third component, even more mysterious, which takes the name of Dark Energy. Very little is known about it. Dark Energy fills about 73% of the Universe, and it is responsible for its accelerated expansion. The origin and nature of Dark Energy are not understood by the scientific community. The picture is completed with 23% of Dark Matter and only 4% of visible matter. We can draw an amazing conclusion: with the development of the Standard Model, we thought that we were able to describe everything in nature. On the contrary, our knowledge is limited to the crumbs of the Universe. Now more than ever, Nature looks mysterious and calls for new explanations!

What is Dark Matter?

Once the existence of Dark Matter is established by the experimental evidence, the natural question is What is Dark Matter made of? However, the answer is not simple. The most natural idea, from a particle physics point of view, is to assume that Dark Matter is nothing but a new kind of particle that we must add to the Standard Model picture. This is consistent with the evolution of particle physics, where new particles have been gradually predicted and discovered to explain the observations. To give an analogy, in the 20s the physical community struggled to understand a nuclear process known as Beta decay, in which a neutron converts into a proton and emits an electron. This process, which was observed in laboratories all over the World, seemed to violate basic physical principles such as the conservation of energy. Indeed, some energy seemed to be lost somehow in the process. An elegant solution was proposed by the Austrian physicist Wolfgang Pauli, who postulated the existence of a new light particle, then named neutrino. The neutrino is also emitted in the decay and carries this missing energy. More than 30 years after his proposal, the first neutrinos were observed by experimental physicists, validating Pauli’s proposal. Nowadays, neutrinos are an important ingredient of the Standard Model. The hope is that something similar could repeat now with Dark Matter! 

Schematic representation of the Beta decay.

Is there some guideline we can follow if we want to propose realistic Dark Matter candidates? Given the lack of direct evidence, we can only rely on some general properties. Dark Matter must be electrically neutral; otherwise it would interact with photons and emit light. It must be a stable particle, which means that it cannot disintegrate, creating new particles (this is a common phenomenon in particle physics. Heavy particles tend to disintegrate, or in technical terms, decay, into lighter ones unless some conserved quantity, such as the electric charge, forbids the process. Most of the Standard Model particles are unstable and decay. On the contrary, electrons and protons are stable particles. If this were not the case, atoms would continuously disintegrate, and no stable form of matter would exist). 

Unfortunately, there is a large set of possible candidates that satisfy these basic properties. Furthermore, the several candidates that have been proposed in the scientific literature have completely different structure and features. I will give a couple of examples to give a taste of the variety. Different groups of physicists believe that Dark Matter could be a new and extremely small and light particle, 10^20 (1 followed by twenty 0s!) times smaller and lighter than the electron. Other groups have proposed the idea that Dark Matter is not a particle but a composite object. As an example, it could be an extremely heavy astrophysical body, such as a Black Hole, tens of times larger than the Sun! The amazing conclusion is that even ideas that look so different can explain the same set of observations. It is not hard to understand that finding the correct answer is a difficult task. 

However, we should not lose hope! The best ideas in physics always came out when everything looked difficult and inexplicable. The active research on Dark Matter in the last decade has led to a proliferation of new ideas on both the theoretical and experimental sides. Many interesting proposals are available, and thanks to fast technological development, experiments are becoming more and more powerful and precise. The hunt for Dark Matter is open, and new results are expected soon! 

Summary of the main Dark Matter candidates proposed in the scientific literature. The diagram gives an idea of the complexity of the topic!

The hunt for Dark Matter

The astrophysical and cosmological observations that we have collected over the years are indirect proof of the existence of Dark Matter. Indeed, we never detected a Dark Matter particle, but we only observed the effect of the presence of Dark Matter on the surrounding environment. To establish the nature of Dark Matter, we must be able to provide a direct experimental signature. A plethora of experiments are actively searching for some trace of Dark Matter, with various strategies specialized for each different theoretical candidate. I will briefly comment on two active research lines. For simplicity, I will only focus on the possibility that Dark Matter is a particle moderately (100-1000 times) heavier than the proton and the neutron. 

The first strategy to detect these kinds of particles is called Direct Detection. The basic idea is quite simple:  a Dark Matter particle coming from Space can occasionally collide with the atoms of the material in the detector. This scattering process gives rise to an electromagnetic signal (typically an electron) that can be directly measured. From the properties of this signal, it is possible to reconstruct the nature of the Dark Matter particle. The main challenge for these kinds of experiments is to distinguish the eventual Dark Matter signal from the similar ones provided by collisions with Standard Model particles, the so-called background. To reduce the background, one typically needs very large detectors, usually located underground. One of the most important experiments is XENON located at the INFN Laboratori Nazionali del Gran Sasso, Italy. It consists of a huge cylindrical detector filled with liquid xenon. Thanks to the special properties of this material, the experimental collaboration has been able to explore different configurations corresponding to different Dark Matter candidates. Unfortunately, no signal has been observed yet!

The incoming Dark Matter particle scatters on the atoms of the detector producing an observable signal. 

An alternative strategy for the search for particle Dark Matter is to produce them at collider experiments. Collider physics has been the main tool to investigate new particles through the 20th century. The heaviest particles of the Standard Model have been discovered in this way. The main idea is the following: electrons or protons get accelerated using strong electromagnetic fields until they almost reach the speed of light. Moving so fast, these particles carry a large amount of energy. When two of them collide, this energy can be released.  The famous equation E=mc^2, provided by Einstein in 1905, explains how an amount of energy E can be converted into a particle (or many particles) of mass m (the conversion factor, c, is the speed of light). Therefore, if the energy released in the collision is large enough, it can be converted into a pair of Dark Matter particles. The main challenge is to provide a large enough amount of energy, which can be achieved by building a powerful enough collider. Currently, the best instrument that we have built is the LHC at CERN, however, has not been able to produce Dark Matter yet. The scientific community is actively discussing the possibility of building a larger and more powerful collider. Stay tuned! 

Schematic representation of the production of Dark Matter from the collision of accelerated Standard Model particles.

Old questions…looking for an answer!

The Universe continuously surprises us with new, apparently unexplainable phenomena. The old question What is matter made of? has been only partially answered. While we have a good understanding of the constituents of ordinary matter we deal with in our lives, we are still ignorant about the composition of our Universe. The identification of the nature of Dark Matter is one of the most exciting challenges for physics in the 21st century. A new generation of physicists is ready to unravel the mysteries of Nature and try to improve our knowledge of the World surrounding us!