V. Motta

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.

You don't hold a ruler right up to an invisible disk; you measure it across the entire universe. Light from a distant quasar, a blazing gas swirling around a supermassive black hole, passes through a lensing galaxy on its way to us.