A long-lived heavy neutrino could have reshaped the early universe’s expansion and tipped the balance toward matter, according to a theory that predicts traces in primordial gravitational waves.

If matter and antimatter had annihilated each other completely after the Big Bang, the universe would have been left with energy and no matter. Explaining why matter survived, when current theories suggest the two should have wiped each other out, remains one of the biggest puzzles in modern physics.

Physicists at the University of Southampton and the Indian Institute of Technology Kanpur have proposed an explanation involving a particle largely overlooked for decades. Known as a “decoupled” heavy neutrino, it is a heavier relative of the neutrino, one of the lightest particles in the universe. Most theories explaining the matter-antimatter imbalance have ignored this heavy relative because its interactions with other particles were considered too weak to have an impact.