Avi Loeb, a Harvard astrophysicist, has proposed 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
According to Loeb, the interaction between antimatter and solar particles could explain the object's unusual brightness and apparent acceleration. He suggests 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 3I/ATLAS may contain antimatter.