* Vacuum has no mass (gravitational attraction). * Normal matter has mass. * Vacuum is composed of matter and antimatter. * Therefore, antimatter has negative mass (negative gravitational attraction).
One of the wikipedia pages seems to support that:
http://en.wikipedia.org/wiki/Virtual_particles "As such, virtual particles are also excitations of the underlying fields, but are detectable only as forces but not particles."
It seems to have something to do with normally existing only for a very short time, and affecting the universe only on a very short range that these virtual particles get away with appearing to not having a mass from the point of view of the rest of the universe. http://en.wikipedia.org/wiki/Virtual_particles