Final answer:
Antimatter does not have negative mass; both matter and antimatter have the same mass, but opposite charges. Neutrons have an antimatter counterpart known as the antineutron. Significant research effort continues toward understanding the imbalance of matter and antimatter within the universe.
Step-by-step explanation:
The answer to the question Does antimatter have a negative mass? is b) No. Antimatter does not have negative mass; rather, particles and their corresponding antiparticles have the same mass. The distinctive feature of antimatter lies in its charge, which is opposite to that of its matter counterpart. For example, protons have a positive charge, while antiprotons have a negative charge. However, both have the same mass. This principle also holds true for neutrons; neutrons have an antimatter counterpart called the antineutron, which also has the same mass as a neutron.
Antimatter particles such as antiprotons can be confined within magnetic fields to prevent them from annihilating with matter. Discoveries such as antihydrogen atoms observed at CERN in 1995 confirmed that antimatter counterparts exist for each particle. Despite the difficulty in storing significant quantities of antimatter due to repulsive forces between similarly charged particles, continued research seeks to understand why there is more matter than antimatter in the universe.