The galaxy is not a smooth lake but a pond with ripples. These ripples are created by ultralight dark matter (particles billions of billions of times lighter than an electron).
Boiling milk develops a trembling skin. By watching its movements, you can guess the strength of the fire beneath the pot and the fat content of the milk. In a similar way, astrophysicists peer into the ancient microwave radiation — the oldest light that has reached us from an era when the universe was only 380,000 years old.