Michael

Inflation occurs before the first second after the Big Bang.

The idea is that vacuum is a scalar quantum field called a Higgs Field. The current vacuum is a higgs field in its lowest energy state. When the Big Bang happened this field is in a high energy state, a false vacuum. As space expands and imediately starts cooling (albeit a little) the field undergoes spontaneous symmetry breaking and drops to a lower energy level, the current vacuum. This means that it has a negative pressure compared to its high energy state.

In the Einstein Field Equations gravity can be explained as a pressure formed by fluid flow, the fluid is spacetime. A negative pressure would act as a form of anti-gravity causing the expansion of the Universe.

Later as the Universe cools more particles, baryons, photons and leptons are formed. As explained these are initially kept in equilibrium as particles decay and recombine. In a second case of spontaneous symmetry breaking particles 'freeze' out' of the Universe. Due to Charge-Parity-Time symmetry violations there a tendency to form matter over anti-matter. And here we are today.