Imagine you’re trying to build the biggest, most awesome LEGO castle ever. You gather all your colorful bricks—the reds, blues, yellows, and greens. You plan your design, start stacking the bricks, and build towers, walls, gates, and secret tunnels. Everything’s going great. But suddenly, something odd starts to happen. Parts of your castle stick together more tightly than they should. Some towers refuse to fall over, no matter how much you poke them. And even stranger, the whole play area you’re building on starts stretching. The towers begin to drift apart—not because you're pushing them, but because the ground underneath is quietly, constantly expanding.

You check your LEGO instructions. Nothing in them explains this. That’s when you notice a strange pattern. It’s as if invisible LEGO bricks are holding your castle together, and an invisible force stretches your whole LEGO world. You can’t see these things, but you know something’s there because of how everything behaves. And that’s when it hits you: you’re not missing any pieces—you just didn’t know most of them were invisible.

In a very simplified way, this is the cosmic puzzle scientists face when trying to understand the universe. We can see all the stars, planets, and galaxies—what scientists call "ordinary matter"—are just visible bricks. They glow, spin, smash, explode, and shimmer across the vast blackness of space. They’re dramatic, photogenic, and spectacular. But here’s the thing: they only make up a tiny slice of the universe—less than 5%.