The universe is vast and full of unknowns, but one of the most perplexing mysteries in modern astrophysics is dark matter. Unlike stars, planets, and galaxies, dark matter does not emit, absorb, or reflect light, making it completely invisible. Yet, scientists believe it makes up around 85% of the matter in the universe. How do we know it exists, and what role does it play in the cosmos?
Even though dark matter cannot be seen, its presence is detected through its gravitational effects. In the 1930s, Swiss astronomer Fritz Zwicky first noticed that galaxies in the Coma Cluster were moving much faster than they should be based on visible matter alone. If only stars, gas, and dust were responsible for the cluster’s mass, the galaxies would have flown apart. This suggested that some unseen mass—what we now call dark matter—was holding them together with its gravitational pull. Later, in the 1970s, Vera Rubin and Kent Ford discovered a similar effect in spiral galaxies. The outer edges of galaxies were rotating at the same speed as their inner regions, defying Newtonian expectations. The presence of a massive, invisible halo of dark matter was the only explanation that made sense. Since then, countless observations of galaxy clusters, gravitational lensing, and cosmic background radiation have reinforced the idea that dark matter is an essential part of the universe’s structure.
Although scientists agree that dark matter exists, its exact nature remains unknown. Several theories attempt to explain what dark matter might be. Some propose that it consists of Weakly Interacting Massive Particles (WIMPs), hypothetical particles that only interact via gravity and possibly the weak nuclear force. If they exist, WIMPs would be everywhere in the universe but nearly impossible to detect. Another possibility is axions, extremely light particles that could behave like a cosmic fluid, influencing galaxy formation and the early universe’s structure. Other theories suggest sterile neutrinos, a heavier version of regular neutrinos that might only interact through gravity, or even primordial black holes, tiny black holes formed shortly after the Big Bang that could account for some of the missing mass.
Despite its elusive nature, researchers are using several approaches to hunt for dark matter. Direct detection experiments, such as those at the XENON, LUX-ZEPLIN, and DAMA/LIBRA experiments, are designed to capture rare interactions between dark matter particles and regular atoms. Particle accelerators like the Large Hadron Collider (LHC) at CERN smash particles together at high speeds in hopes of producing dark matter or clues about its nature. Astronomical observations analyze distant galaxies and gravitational lensing (the bending of light by gravity) to indirectly study the effects of dark matter. Each new experiment brings us one step closer to solving the puzzle.
Dark matter is more than just an astronomical curiosity—it is fundamental to the universe’s structure. It shaped galaxies, allowing them to form and remain stable over billions of years. Without dark matter, the Milky Way and other galaxies might not even exist. Moreover, understanding dark matter could revolutionize physics. If scientists discover what it truly is, it might lead to a new understanding of gravity, quantum mechanics, or even entirely new physics beyond the Standard Model. For now, dark matter remains one of the greatest unsolved mysteries in science, but with ongoing research, we may be on the verge of a discovery that could reshape our understanding of the universe itself.