Most of the Universe Is Invisible

Everything we have ever seen — stars, planets, gas, people — makes up only about 5% of the universe. Every galaxy in every telescope photo, every comet, every grain of sand on every beach, the air in your lungs and the screen you are reading this on: all of it belongs to that thin slice. The other 95% is made of two things scientists can measure but cannot yet explain. They gave them the most honest names they could think of: dark matter and dark energy.

How we know something is missing

Astronomers did not invent the dark universe because they wanted a mystery. They were pushed into it by measurements that refused to add up.

In the 1930s, an astronomer studying a cluster of galaxies noticed that its members were moving far too fast. Based on the light they gave off, there was not nearly enough mass to hold the cluster together. It should have flown apart long ago. Decades later, researchers measuring how quickly stars orbit the centers of spiral galaxies found the same puzzle. Stars on the outer edges were moving about as fast as stars near the middle, which only makes sense if a large amount of unseen mass is wrapped around each galaxy like an invisible halo.