In a mere fraction of a second, the universe expands proportionally more than it will in the next 15 billion years. At the time of inflation, the universe is so hot that matter exists only as a hot plasma of fundamental particles. But the plasma isn't entirely smooth. Inflation stretches tiny variations that exist on subatomic scales to astronomical sizes. This makes the plasma lumpy, like bad gravy.
Gravity tries to make high-density lumps contract.
But the plasma fights back. Photons of light bounce around the plasma like light in a hall of mirrors, generating a collective pressure that keeps matter from compressing. For hundreds of thousands of years, this fight between gravity and light sends sound waves racing through the plasma at about half the speed of light. During this era, the plasma is so hot and dense that electrons float freely, scattering light before it can travel far. The universe is opaque.