The possibility of repulsive gravity arises because, according to Einstein's theory of general relativity, gravitational fields are produced not just by energy or mass densities, but also by pressures. The direction of the field caused by pressure is what you would probably guess: a positive pressure - the kind that we normally see - produces an attractive gravitational field. But the peculiar state of matter that I'm talking about produces a negative pressure, which you might also call a suction. It is in fact a very large negative pressure, resulting in a repulsive gravitational field which is stronger than the attractive field produced by the mass density of the matter. The result is a net gravitational repulsion, which is the driving force behind inflation.