Cosmic inflation is the theory that in the first tiny fraction of a second the universe expanded exponentially, ballooning by a factor of trillions of trillions. Inflation explains why the cosmos looks so uniform and flat, and how quantum fluctuations stretched into the seeds of galaxies. Its predicted signature, a faint swirl pattern in the polarization of the cosmic microwave background from primordial gravitational waves, has not yet been detected, only tightly constrained.