That would be normal positive pressure. A model where a gas is expanding and pushing things with it. That's not what's happening here. Negative pressure means that energy is required to expand the vacuum - rather than energy required to compress it. Note that vacuum energy is positive: if a vacuum expands and its energy density remains constant - as it must, assuming vacuum is vacuum and you can't dilute a vacuum - then that implies more total energy. The other energies (matter and radiation) have the obvious property that as space expands, their energy densities must decrease. Vacuum energy has what seems the extraordinary property that this does not happen and, as space expands, the total vacuum energy goes up and up. As I understand it, that is where we reach the mystery of dark energy.