Cosmic inflation is a proposed period, lasting a minuscule fraction of a second right after the Big Bang, during which the universe expanded exponentially, growing by a factor of trillions upon trillions almost instantly. It was put forward in the early 1980s to solve several puzzles that the plain Big Bang model on its own could not explain.
One of those puzzles was why the universe looks so uniform in every direction, even in regions that, without inflation, would never have had time to interact and reach the same temperature. Inflation solves this by stretching a tiny, uniform patch to enormous size, smoothing everything out before gravity and other forces had a chance to create large differences.
Inflation also predicts that tiny quantum fluctuations present in that first instant would get stretched to cosmic scales, becoming the seeds for all future structure. Those predicted patterns closely match what is actually observed in the Cosmic Microwave Background, which is the strongest evidence in favor of inflation having happened.
The engine behind inflation is thought to be a hypothetical field called the inflaton, whose energy briefly dominated the universe and drove the expansion before decaying into the particles that fill the cosmos today. In that fraction of a second, commonly estimated between 10 to the negative 36 and 10 to the negative 32 seconds after the Big Bang, the universe is thought to have doubled in size dozens of times over, growing by a factor of at least 10 to the 26. Despite strong indirect evidence, inflation remains a framework rather than a single confirmed theory, since dozens of different versions have been proposed and physicists are still searching for a direct, unambiguous signature, such as a specific pattern of polarization in the Cosmic Microwave Background, that would confirm exactly how it happened.