X. Du

In the kitchen, when two broths are mixed in one pot, the resulting flavor is new, unlike either of the originals. A similar trick is possible with sound: two waves meeting on the surface of a crystal give birth to a third, with a frequency that neither of them had.

In the 1930s, Fritz Zwicky first noticed that galaxies in clusters were moving too fast — something invisible was holding them. Later, Vera Rubin measured the rotation of spiral galaxies, confirming that space is permeated by dark matter.