A negative density false vacuum is a phenomenon that occurs inside a star that is 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 draws surrounding matter into the Continuum.

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")