Antimatter is often described as the "opposite" of matter because it consists of particles that have opposite electrical charges compared to their corresponding particles in regular matter. For example, an antielectron (positron) has a positive charge, whereas an electron has a negative charge. However, when we talk about the mass of particles, both matter and antimatter have positive mass.
In theoretical physics, the concept of negative mass exists, but it is purely speculative and has not been observed in nature. Negative mass would have peculiar properties, such as accelerating in the opposite direction of an applied force. However, it is important to note that negative mass is currently only a theoretical construct and has not been confirmed to exist.
Regarding the use of antimatter in warp drives or wormhole creation, these ideas are primarily within the realm of science fiction and speculative theories. While the concept of warp drives and wormholes has been explored in science fiction, the scientific community has not yet developed a practical and feasible method for their realization. The physics behind these concepts, if they were possible, would require exotic forms of matter and energy that are not currently understood or accessible.
It's worth emphasizing that our current scientific understanding suggests that achieving warp drives or traversable wormholes is highly unlikely based on our knowledge of physics. While scientific exploration and discoveries may lead to new insights in the future, at present, these concepts remain speculative and do not have a basis in established scientific principles.