Harvard astrophysicist Avi Loeb has suggested that the interstellar object 3I/ATLAS could be composed of antimatter, a form of material that annihilates ordinary matter and releases nearly total energy on contact.
Antimatter is "the most efficient fuel" in the universe
Loeb argues that the interaction between antimatter and solar particles could account for the object's unusual brightness and apparent acceleration. He also proposes that if 3I/ATLAS truly contained antimatter, its passage through the solar wind might generate detectable gamma-ray or X-ray emissions, which could be observed by NASA's Fermi Gamma-Ray Space Telescope.
Author's summary: Avi Loeb suggests 3I/ATLAS contains antimatter.