Far, far away, when the universe was only two billion years old, a necklace of galaxies hung in one corner of it. They are strung on invisible filaments that astronomers call the cosmic web. These filaments, like strings, carry gas across the universe, gas from which stars are born. Eighteen such beads were studied with large telescopes to understand how they change.

Some beads are soft and fluffy, like taffy: they have lots of gas and stars are scattered. Others are already hard and dense, like hard candies: the gas is gone, and the stars have clumped together. Astronomers lined up all the beads and saw that the transition from taffy to candy is gradual. The larger the galaxy, the less gas it has and the more tightly packed it is.

Why does this happen? The filament on which a bead hangs first feeds it with gas. As long as food arrives, the galaxy forms stars and remains fluffy. When the gas flow dries up, the fuel runs out, and the galaxy contracts under its own gravity, like a piece of taffy that has been kneaded in the hand for a long time. So a necklace of soft taffies eventually becomes a string of hard candies. Today, in the centers of galaxy clusters, exactly such candies live: giant, quiet stellar cities that have not formed new stars for a long time.