A Mystery in the Sky

Have you ever tried to count the stars in the night sky? If you have, you probably had to give up pretty fast—there are just too many! Actually, with the naked eye you mostly see only the stars that surround us in our home galaxy—the Milky Way. But the Milky Way is only one of billions and billions of galaxies in the universe. Each galaxy contains billions of stars along with huge amounts of gas and dust! However, the matter contained in all these galaxies turns out to be only 15% percent of the total matter in the universe. The remaining 85%, scientists believe, is made of invisible stuff that they call dark matter [1].

As dark matter is invisible, how do scientists know it is there? The key is that dark matter has mass and, therefore, has a gravitational pull—the same way as the Sun’s gravity attracts the Earth and the Earth’s gravity keeps us from floating off into space. So, even though scientists cannot observe dark matter with their telescopes directly, they can look for its gravitational footprints. To discover how these gravitational footprints look, check out this Frontiers for Young Minds article.

What is “Normal” Matter?

Scientists have a lot of ideas about what dark matter could be, but before we dive into that, we will explain a few things about “normal”, visible matter. You, us, and everything around you, from atoms and cells to planets and galaxies, is made of the same tiny elementary particles arranged in different combinations. These particles are the smallest building blocks of nature and form a “family” that scientists call the Standard Model. Just because it sounds more fun, we are going to name this particle family the Standardsons!

The Standardsons are a very big family (Figure 1). There are two types of particles. Matter particles are what visible matter is made of. Matter particles include the “Quark Gang” consisting of six quarks: Up, Down, Charm, Strange, Top, and Bottom, and the “Lepton Clan”: the three brothers Electron, Muon, and Tau, and their little companions Electron Neutrino, Muon Neutrino, and Tau Neutrino. Quarks do not like to move separately, so you always meet a group of at least two of them, “talking” with each other by throwing interaction particles called gluons back and forth. Apart from gluons, other interaction particles are the fast and furious photon (nickname Photonetto) and the three weak bosons: W Plus, W Minus, and Z. They are called weak because they are quite heavy and do not like to move much. Photonetto, on the other hand, is extremely light (in fact, it is massless!) and is the champion of speed. Photons interact with particles that have electric charge, and this is how they help people to see things as they reach their eyes, cameras, or telescopes. The oldest of all the Standardsons is the Grandpa Higgs. He is very, very heavy and gives mass to all the matter particles and the weak bosons in the Standard Model. Neutrinos might not get their masses from the Higgs boson, because they are much much lighter than the other massive particles in the Standard Model, but that is a story for another time…

How is Dark Matter Different?

But why is it that dark matter cannot also be made of these elementary particles? Building dark matter requires a particle with some special properties. First, this particle should have mass. Second, it should live a long, long time—at least as long as the age of the universe, which is 13.8 billion years. Third, it should not have any electric charge, because otherwise people would see it with telescopes thanks to the photons. Only three members of the Standardsons family, the three neutrinos, have these properties. However, neutrinos are extremely light and tend to move too fast to be responsible for all the dark matter. You see, judging by its gravitational footprints, dark matter seems to move slowly [2]. That could be because it is much heavier than the members of the Standardsons family. However, recently more scientists are thinking that dark matter, although still moving very slowly, could be much lighter than Grandpa Higgs. So, it looks like we need to add one new invisible member to our article family…or maybe more than one!

The Dark Matter Family

Some scientists believe that dark matter could be only the tip of the iceberg—there might be a whole hidden world with its own kinds of particles and forces between them [3]! Scientists call this invisible world the Dark Sector. You can think of the Dark Sector as a family, similar to the Standardsons, but way more exotic and mysterious. We will call this family the Darkfields.

The Darkfields family could be as big as the Standardsons, or even bigger! It could contain several matter particles of different kinds, including a stable particle: the cute Oscurina (Figure 2). It is Oscurina’s gravitational footprints that we see in the universe. Among the Darkfields, there might also be a Dark Neutrino, possibly a very heavy one. Scientists call this heavy dark neutrino a “sterile neutrino” or a “heavy neutral lepton”. Similar to the Standard Model, there might be several types of dark forces acting between the dark particles. For example, there could be a Dark Photonetto, a dark version of the standard photon. The Darkfields might also have a dark counterpart of Grandpa Higgs, the Dark Higgs, which could give dark particles their masses. And these are just a few examples!

The Standardsons and the Darkfields are like two families that live on different continents, speak different languages, and have never met each other. They would not even know of each other’s existence. This is how the Standard Model and the Dark Sector might coexist. But what if some sneaky particles found a secret portal to the other place, and from time to time visit the other family? Maybe some of them actually enjoy playing with members of the other family and are even secretly friends with them?

Finding the Dark Sector

Scientists believe that there might be several types of portals between the Standard Model and the Dark Sector (Figure 3) [4]. First, the photon and the dark photon might like to visit each other through a so-called vector portal. Second, Grandpa Higgs might be secretly friends with Dark Higgs, connected with him through a Higgs portal. And finally, there might be a neutrino portal through which neutrinos from the Standard Model can play with dark neutrinos.

If such portals exist, scientists could try to catch one of these sneaky particles and learn about the Darkfields family from them. This is exactly what scientists are doing! For example, at CERN in Geneva, Switzerland, scientists are using the Large Hadron Collider, a very powerful machine, to produce all kinds of elementary particles from the Standard Model. Some members of the Darkfields family might be hiding between these particles, and scientists are hoping to catch them. It is not an easy task though—it is like trying to find a tiny needle in a haystack! But a true scientist is not scared of a challenge!

Another example is the Deep Underground Neutrino Experiment (DUNE), which is currently being built in the United States. At DUNE, scientists are going to produce huge amounts of neutrinos to study them and their role in the universe. Among these neutrinos, a dark neutrino might be sneaking around. Will the scientists at DUNE be able to catch it? Stay tuned!

Experiments at CERN and DUNE are just two examples of how scientists can look for the Darkfields with experiments built on Earth. But this is not the only way to search for dark particles! Astroparticle physicists are constantly coming up with new ideas. For example, a group of scientists is trying to use streams of stars in our galaxy to learn about dark matter. If you are curious about this method, check out this Frontiers for Young Minds article.

The Big Questions…

Why do scientists care so much about what dark matter is in the first place? One reason is because it would be cool to know what most of the matter in the universe is made of, but that is not the only reason. Understanding the dark world would help us to better understand our own world and hopefully give us hints about other very important open questions like, for example, why matter exists at all!

Do we all share our universe with a hidden dark world? Hopefully, science will help answer this question one day. But for now, the next time you look at the night sky, think about all the invisible particles that might be hidden up there…

Glossary

Dark Matter: ↑ Invisible stuff that makes 85% of matter in the universe and interacts mostly (or possibly only) through gravity. Scientists do not know yet what dark matter is made of.

Gravity: ↑ One of the fundamental forces in the Universe. It is gravity that, for example, makes things fall to the ground and keeps the Earth and other planets orbiting the Sun.

Elementary Particle: ↑ A particle that is not built of any smaller particles. It is a fundamental building block of nature.

Interaction Particle: ↑ A particle that transmits a force (like a push or a pull) from one matter particle to another, and in this way helps them to communicate with each other.

Electric Charge: ↑ A basic property of elementary particles. Electric charge can be positive or negative. Only particles that have an electric charge can interact with photons.

Stable Particle: ↑ A particle that does not get transformed (decay) into lighter particles over time.

Astroparticle Physicist: ↑ A scientist who studies what the Universe is made of by looking at particles coming from space, like from the Sun or faraway galaxies.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This article was written in connection with the outreach project inVISIBILI, which was supported in part by the European Union under the Marie Sklodowska-Curie grant agreement no. 860881-HIDDEN and the ERC-2020-STG grant agreement no. 950692-CosmicAntiNuclei. The authors acknowledge the Department of Physics and Astronomy “Augusto Righi” (DIFA)–Alma Mater Studiorum Università di Bologna and INFN Bologna for the invaluable support provided throughout the project. The authors also extend their gratitude to all the scientists, schools, teachers, and children for their enthusiastic participation, which made the inVISIBILI project possible. ML was funded by the European Union under the Horizon Europe’s Marie Sklodowska-Curie project 101068791—NuBridge. SP was funded by the European Union under the Horizon Europe’s Project 101201278—DarkSHunt - ERC - 2024 ADG. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. EM was supported in part by the INFN program on Theoretical Astroparticle Physics and by the European Union—NextGenerationEU under the National Recovery and Resilience Plan (NRRP), D.M. 630/2024 (CUP J33C24001440009). GF received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme grant agreement 101019620 (ERC Advanced Grant TOPUP).

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