A positive pressure produces an attractive gravitational field... Positive pressures are just sort of normal pressures and attractive gravity is norm… - Alan Guth

In short what I like to say is that the Big Bang says nothing about what banged, why it banged, or what happened before it banged. It really has no bang in the Big Bang. It is a bangless theory, despite its name.

Inflation takes advantage of this possibility... to let gravity be the repulsive force that drove the universe into the period of expansion that we call the Big Bang. In fact, when one combines general relativity with conventional ideas, now, in particle physics there really is a pretty clear indication, I should say, not quite a prediction... that at very high energy densities one expects to find states of matter which literally turn gravity on its head and cause gravity to become repulsive.

Inflation is a prequel to the conventional Big Bang theory. ...It does provide a theory of the propulsion that drove the universe into this humungous episode of expansion which we call the Big Bang.