For the first time, scientists have determined how most of the antimatter in the Milky Way, our home galaxy, forms. The researchers suggest that the production of most antimatter is via the collision of subatomic particles called protons.

Antimatter is the mirror image of ordinary matter. Unlike matter, however, antimatter annihilates—or vanishes into pure energy—when it comes into contact with regular matter. This annihilation is the equivalent of taking one gram of matter and converting it into energy, which would be equal to the energy of a mushroom-shaped atomic cloud explosion!

For this reason, antimatter cannot naturally occur on earth, and must instead be created in particle accelerators such as at the Large Hadron Collider (LHC), where scientists smash subatomic particles together to create antimatter and study it.

Despite its rarity, the universe contains antimatter. There are even entire antimatter galaxies, where antimatter is everywhere and matter is rare.

The question of where antimatter comes from has puzzled scientists for decades. For more than 50 years, they suspected that much of the antimatter in our Milky Way originates in cosmic ray interactions with interstellar matter, but no definitive proof has existed until now.

Cosmic rays are made up of energetic charged particles that are accelerated in supernova explosions and other energetic phenomena from the cosmos. When cosmic rays enter the Milky Way from the outside or are born inside the galaxy, they slam into interstellar gas and dust inside giant molecular clouds—vast reservoirs of gas and dust where new stars form.

Using a combination of computer modeling and observations with the Fermi gamma-ray space telescope, scientists have now confirmed for the first time that collisions of cosmic ray protons on the gas and dust inside giant molecular clouds explain most of the observed antiproton fluxes—or flow—measured by the AMS-02 experiment on the International Space Station.

The result is published in the journal Physical Review Letters, and will help unravel the mystery of how some of the universe’s most extreme phenomena take place.

“This is a breakthrough measurement,” said Stefan Funk, an associate professor of physics and Kavli fellow at the University of California, Santa Barbara. “The data and the analysis provided by the AMS-02 team are absolutely fantastic.”