Upcoming telescopes could shed light on dark matter – astronomers are looking for these ‘fingerprints’ of the elusive substance

NASA’s plans to return astronauts to the Moon through the Artemis program and ultimately send humans to Mars highlight just how far space exploration has come. Yet while the Moon and Mars remain compelling destinations filled with scientific mysteries, looking beyond our solar system raises even deeper questions about the universe itself. Among the biggest of those mysteries is matter – the substance that makes up everything around us. Surprisingly, most of the matter in the universe is invisible, and astronomers still do not know what it is. Physicists estimate that about 85% of all matter is made of something we cannot see, touch or directly detect. This elusive substance is known as dark matter . It doesn’t emit light like stars or galaxies. The only reason scientists know it exists is because of its gravity. Galaxies rotate too fast to be held together by just the matter that can be seen. Light bends more strongly than expected as it travels through space. Galaxies within clusters fly past one another much faster than they should based on their visible mass alone. Based on data from across the cosmos, scientists keep coming to the same conclusion: There is something out there that cannot be seen, but whose presence is unmistakable. It’s a question that has intrigued astronomers like us for more than 50 years . So what is dark matter, and why does it matter? A missing piece of the cosmic puzzle Everything in our everyday world is made of atoms, which are combinations of protons, neutrons and electrons. These particles form stars, planets, people and everything you see. Dark matter, scientists believe, is fundamentally different. It is likely made of entirely new kinds of particles yet to be discovered. Understanding what those particles are would fill a major gap in the scientific understanding of physics. But the importance of dark matter goes far beyond particle physics. Dark matter played a crucial role in shaping the universe . Shortly after the Big Bang that kicked off the birth of the universe, it acted as a kind of gravitational scaffolding, helping ordinary matter clump together to form the first galaxies and stars. Even today, it acts as the invisible glue that holds galaxies together. In other words, without dark matter, the universe as you know it might not exist. Looking for invisible matter Because dark matter does not emit light, scientists must search for it indirectly. One promising approach is to look for the signals it might produce when its particles collide and destroy each other through a process known as annihilation . This idea may sound exotic, but it has a familiar analogy. In medical imaging, devices such as positron emission tomography scanners, or PET scanners for short, detect radiation produced when particles of antimatter – positrons – annihilate with electrons, which are normal matter. Antimatter is just a form of matter made of particles that have the same mass as ordinary matter, but opposite charges and quantum properties. The annihilation signals in PET scanners allow doctors to map cancerous tissues inside the human body . Scientists hope something similar could happen with dark matter. If dark matter particles annihilate with each other, they may produce high-energy radiation called gamma rays. These gamma rays could act as fingerprints , revealing where dark matter is concentrated and its properties. As astrophysicists who study gamma rays, we and our collaborators use space-based telescopes to search for these signals. A mysterious signal at the heart of our galaxy One of the most powerful tools for this search is NASA’s Fermi Large Area Telescope , known as Fermi-LAT, which has been observing the gamma-ray sky since 2008. Gamma rays are the most energetic form of light, and they are produced by some of the universe’s most extreme phenomena. For years, Fermi has detected an unexplained glow of gamma rays coming from the center of the Milky Way. Based on gravitational observations such as galaxy rotation curves, stellar motions, and the bending of light , combined with cosmological simulations, astrophysicists expect this region to be extremely rich in dark matter, making it an intriguing place to look for annihilation signals. Could this glow be evidence of dark matter? Possibly. But there’s a complication: The center of our galaxy is also crowded with more conventional astrophysical gamma ray sources, such as rapidly spinning neutron stars, which are produced from the collapse of massive stars. These objects can produce gamma rays that mimic the expected signal from dark matter . At the moment, scientists cannot say for certain what is causing the emission. The signal could be a breakthrough, or it could be something more ordinary. Clues from smaller galaxies To help resolve this mystery, researchers also study much smaller systems, known as dwarf galaxies , which orbit the Milky Way. These galaxies contain dark matter but relatively few other sources of gamma rays, making them cleaner environments to search for dark matter-related signals. So far, no definitive detection has been made.