Take a moment and look up at the night sky. All the stars, planets, and galaxies you see belong to a glittering cosmic tapestry. But here’s the surprise: all that we can see—all that is made of atoms—is only 5% of the universe. What is left? It’s made up of things we can’t see or touch: dark matter and dark energy. These two mysterious constituents make up about 95% of the universe. They don’t absorb light, they don’t emit it, and yet they fill the universe. It’s like discovering that most of the ocean is not visible, and we’ve only been seeing the foam on the surface. So what are dark matter and dark energy? And how do we even know they exist?Let’s dive in.
The Case of the Missing Mass: What is Dark Matter?
The origin of dark matter begins in the 1930s, when Swiss astronomer Fritz Zwicky noticed something peculiar about the Coma Cluster, a cluster of galaxies. The galaxies were moving so fast that they would have been torn from one another, considering the mass we could see. But they weren’t. Something unseen was holding them together. Then, in the 1970s, astronomer Vera Rubin took a measurement of galaxies’ rotation curves and found the same bizarre effect: stars near the edges of galaxies were also traveling at the same velocity as stars near the center, defying Newton’s laws of motion. This led scientists to believe that there’s some kind of invisible matter that surrounds galaxies that provides the added gravity needed to keep everything bound together.This invisible stuff was called dark matter. It doesn’t interact with light or electromagnetic forces, so we can’t see it directly. But it does have mass, so it does have gravitational pull. Think of it as the scaffolding of the universe—without it, galaxies might never have formed, and we certainly wouldn’t be here. And what is it made of? That’s anyone’s guess. Some have suggested exotic particles like WIMPs (Weakly Interacting Massive Particles) or axions. Others speak of changing gravity itself. But so far, dark matter has managed to stay, well, in the dark.
The Great Expansion: What is Dark Energy?
While dark matter holds the universe together, dark energy is pushing it apart. In 1998, two separate teams of astronomers were monitoring distant supernovae, trying to measure the slowing of the expansion of the universe as a result of gravity. But what they learned amazed everyone: the expansion wasn’t slowing—it was accelerating. That discovery won them a Nobel Prize, and gave the world its first glimpse of the phenomenon of dark energy. It’s a mysterious force that seems to be baked into the very nature of space itself, speeding galaxies apart from one another at an ever-growing acceleration. Dark energy is now thought to make up around 68% of the universe. Nobody is quite sure what dark energy is. Some think it’s connected with the energy of nothingness—what scientists refer to as the cosmological constant, a notion that Einstein himself came up with (and later discarded). Others propose it could be an active field, changing over time. Whatever its actual nature, dark energy controls the ultimate destiny of the universe. If it continues to fuel accelerating expansion, the galaxies will stretch ever farther from each other until the dark skies are empty of sight. That is the “Big Freeze” perspective, in which the universe becomes cold and desolate.
Why It Matters?
You might ask: if we cannot see and understand dark matter and dark energy, then why should we care?Because knowing them might unlock the universe’s deepest secrets. They might show us new physics, revolutionize our concept of space and time, and even give us new technologies. It’s like standing at the threshold of a grand discovery, aware that the puzzle is greater than we could have ever imagined. Astronomy has always been about seeing the unseen—peering beyond what the eye can catch. With dark matter and dark energy, we’re faced with the ultimate challenge: to uncover the invisible, to understand what the universe is truly made of. And perhaps, in doing so, we’ll learn a little more about our place in it.
