The conventional Big Bang theory says nothing about where all the matter came from. The theory really assumes that for every particle that we see in … - Alan Guth

It is said that there’s no such thing as a free lunch. But the universe is the ultimate free lunch.

A very plausible choice for when inflation might have happened would be when the energy scales of the universe were at the scale of grand unified theories... which unify the weak, strong and electromagnetic interactions into a single unified interaction. ..we're talking about energies which are about 10¹⁶ times the equivalent energy of a proton mass. ...the initial patch would only have to be the ridiculously small size of about 10^-28 cm across to be able to lead ultimately to the creation of everything that we see on the vast scale of which we see it.

A positive pressure produces an attractive gravitational field... Positive pressures are just sort of normal pressures and attractive gravity is normal gravity, so normal pressures produce normal gravity, but it is possible to have negative pressures, and negative pressures produce repulsive gravity, and that's the secret of what makes inflation possible.