Avi Loeb, a Harvard astrophysicist, has proposed that the interstellar object 3I/ATLAS could be composed of antimatter, which is "the most efficient fuel" in the universe.
Antimatter is "the most efficient fuel" in the universe
Loeb suggests that the interaction between antimatter and solar particles could explain the object's unusual brightness and acceleration. He notes that if 3I/ATLAS contains 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 proposes antimatter composition for 3I/ATLAS.