A thought popped into my head recently about theoretical objects with negative mass...

Ok, so as I understand it, although a theoretical object with negative mass would technically gravitationally repel other objects, the gravity from objects with positive mass would still attract it, and thus overall gravity would pull it towards a planet or other celestial body since its own repulsion would be negligible in comparison.

HOWEVER, in an atmosphere I would expect it to still rise due to bouyancy, since it would necessarily be of very low density, negative in fact (assuming that the absolute value of its mass was more than the mass of its containment vessel). On the moon, however, there is no atmosphere, but having negative density the object would still be less dense than the vacuum surrounding it, so would it still float? I'm thinking it probably wouldn't because vacuum probably doesn't act like a fluid, but I'm not at all sure.