If you get a dense quantum gas cloud cold enough, you can see right through it. This phenomenon, called Pauli blocking, happens because of the same effects that give atoms their structure, and now it has been observed for the first time.

“This has been a theoretical prediction for more than three decades,” says Amita Deb at the University of Otago in New Zealand, a member of one of three teams that have now independently seen this. “This is the first time this been proven experimentally.”

Pauli blocking occurs in gases made up of a type of particle called a fermion, a category that includes the protons, neutrons and electrons that make up all atoms. These particles obey a rule called the Pauli exclusion principle, which dictates that no two identical fermions can occupy the same quantum state in a given system.