Antimatter is a type of matter that has the same properties as regular matter, such as mass and energy, but with the opposite charge. For example, an antiproton has the same mass as a proton, but with a negative charge instead of a positive charge. This is what makes it "anti" matter.
There is currently no evidence or theoretical basis to suggest that antimatter has negative mass. In fact, the concept of negative mass is still largely speculative and has not been observed in any form of matter, including antimatter.
While negative mass remains a topic of scientific debate, there is currently no known way for negative mass to exist in our universe. This is because the laws of physics, specifically the laws of gravity, do not allow for the existence of negative mass.
If antimatter did have negative mass, it would likely behave in unpredictable and chaotic ways. For example, it could potentially repel regular matter instead of being attracted to it by gravity. However, since this is purely speculative, it is impossible to accurately predict the exact consequences of negative mass.
While there have been some theoretical studies and thought experiments exploring the concept of negative mass, there are currently no experiments being conducted to directly observe or create negative mass. This is due to the lack of evidence and understanding of how negative mass would behave in our universe.