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How did Democritus explain the origin of the universe without a purposeful designer?

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Bronze bust of Epicurus with a full beard and draped clothing, displayed on a round inscribed base at the National Archaeological Museum of Naples. — Atomism Epicurus, bronze bust from a Greek original, c. 280–270 bce; in the Museo Archeologico Nazionale, Naples.

Democritus explained the origin of the universe through a purely mechanical system, explicitly rejecting the need for an intelligent designer or purposeful cause. His cosmology was built upon the existence of two fundamental principles: infinite, eternal atoms and the Void, which is an infinite vacuum.1

According to Democritus, motion is also eternal and uncaused. He posited that atoms originally moved in all directions in a state of constant vibration. Through these random collisions, a whirling movement emerged, which naturally brought similar atoms together to form larger bodies and worlds.1

This process occurred not by design, but by necessity—a manifestation of the inherent nature of the atoms themselves. Because both atoms and the void are infinite in extent, Democritus concluded that there must always have been an infinite number of worlds in various stages of growth and decay, all arising from these mechanical interactions rather than divine intervention.1