But if you have negative gravitational mass, all of that changes. If antimatter actually anti-gravitates, falling up instead of down, then gravity sees it as though it were made of anti-mass or anti-energy. Under the laws of physics that we currently understand, quantities like anti-mass or anti-energy don’t exist. We can imagine them and talk about how they would behave, but we expect antimatter to have normal mass and normal energy when it comes to gravity.
If anti-mass does exist, though, then a slew of great technological advances, imagined by science-fiction writers for generations, would suddenly become physically possible.