A negative density false vacuum was a phenomenon that occurred inside a star that was imploding due to spatial disruptions within the Q Continuum, such as those that occurred during the Q Civil War of 2373. Just before the supernova, the false vacuum drew surrounding matter into the Continuum.
In 2373, the temporarily depowered female Q showed the crew of the USS Voyager how to enter the Continuum through this false vacuum. (VOY: "The Q and the Grey")