When you look up at the night sky, you mostly see darkness dotted with a few stars. But, what if I told you there’s a lot more out there, hidden beyond our sight? This dark expanse is not just a vacuum as we’ve been taught. It’s actually filled with a mysterious substance known as “dark matter,” a critical piece of the puzzle in understanding what the universe is truly made of.

The visible matter we can observe—with all our telescopes and instruments—is just a tiny fraction of what’s out there. In fact, there’s about six times more invisible matter, or dark matter, lurking in the cosmos. Think of it as the unseen ocean beneath the waves we can see.

But how do we know dark matter exists if we can’t see it? Evidence first popped up in the 1930s when Swiss astronomer Fritz Zwicky noticed galaxies in clusters moving so quickly that they should have been torn apart if there wasn’t more mass than what he could see. He coined this unseen mass “dark matter” and though many dismissed him at first, the concept stuck.

Fast forward to the 1970s, Vera Rubin observed that stars on the outskirts of galaxies move at similar speeds to those closer to the center. This was strange, as you’d expect stars further out to move more slowly. The gravitational pull of visible matter alone couldn’t explain this, suggesting an unseen mass was influencing these stars. This unseen mass formed a sort of halo around galaxies, hinting strongly at the existence of dark matter.

So, what is dark matter? It isn’t conventional matter like dim stars or black holes. Observations have basically ruled out these possibilities because ordinary matter, unlike dark matter, clumps together and doesn’t distribute evenly around galaxies.

To figure out what dark matter could be, scientists propose it must have specific properties: it’s dark (doesn’t emit light), it interacts through gravity, it doesn’t interact much with itself, it’s cold (not moving too fast), and it’s stable over billions of years. With these criteria, two main candidates have emerged: WIMPs (Weakly Interacting Massive Particles) and Axions.

WIMPs are appealing because their properties align with existing models in particle physics, specifically supersymmetry, which predicts particles that interact weakly with other forces and fit the description of dark matter. However, despite intense searches, we’ve yet to find solid evidence for WIMPs.

Axions address another puzzle in physics—the strong CP problem related to the behavior of neutrons in electric fields. They’re lightweight and numerous, and if they exist, they could also account for dark matter.

Experiments are ongoing at various locations like CERN, with researchers seeking to uncover these elusive particles. However, no definitive proof has been found so far, which leaves the mystery open.

Dark matter remains one of the most exciting and challenging fields in modern astrophysics. While we have numerous theories and leads, the true nature of this hidden universe awaits discovery. This enigma continues to drive physicists to innovate and explore, promising new insights and breakthroughs for decades to come.