What is Dark Matter?
It's wonderful to explore this cosmic mystery with you. Before we dive into the invisible scaffolding of our universe, I'd love to know what sparked your interest in dark matter today, and how much prior background in physics or astronomy you have.
I'm a beginner who just saw a cool space documentary.
It's so exciting to hear that a documentary got you hooked—space has a way of doing that! Let's start with the absolute fundamentals of what makes this mystery so captivating, starting with what we are actually looking at.
Your path
Observational Evidence and Rotation Curves
Leading Dark Matter Candidates
Role in Cosmic Structure
The Five Percent Illusion
When you look up at a clear night sky, it feels like you're looking at the entire universe. Every twinkling star, every glowing nebula, and every swirling galaxy seems to fill the vast emptiness. But here's the ultimate reality check: everything we can see, touch, and interact with is just a tiny fraction of what is actually out there.
All the stars, planets, gas clouds, and even ourselves—made of what scientists call —account for a mere 5% of the total universe. The rest is divided into two invisible mysteries: dark energy, which makes up about 68%, and our focus for today, dark matter, which commands roughly 27% of the cosmos.
A pie chart showing the composition of the universe divided into dark energy, dark matter, and visible matter.
I've sketched out a visual pie chart above so you can see just how much dark matter dominates the actual "stuff" of the universe compared to the ordinary matter we are made of. It outweighs visible matter roughly five to one!
So why do we call it "dark"? It isn't because it's a dark color like black paint. It is because it is completely, utterly invisible. It doesn't absorb, reflect, or emit light of any kind. If you shone a flashlight at a clump of dark matter, the light would pass right through it as if nothing was there. The only reason we know it exists is because its massive tugs on the visible stars and galaxies around it.