For half a century, astronomers have been unable to find the source of the most energetic rays in our universe. But now, after detecting them for more than a decade, South American researchers say cosmic rays come from outside the Milky Way.
The rays, consisting of energetic sub-atomic particles, may be born from violent events millions of light years outside our galaxy, evidence suggests,
The discovery reported in the journal Science follows 12 years of data collection by the Pierre Auger Observatory in Argentina.
The observatory has 1,600 particle detectors spread in a hexagonal grid over 1,160 square miles to detect cosmic rays,
Cosmic rays are tiny fragments of atoms, and they fill space with varying amounts of energies.
Ultra high-energy cosmic rays have energies millions of times greater than any rays detected on Earth - and up until now, no one has known where they come from.
Lead scientist Professor Gregory Snow, from the University of Nebraska-Lincoln, said: 'There have been other pieces of evidence, but I would say this paper really confirms that most of the highest energy cosmic ray particles are not coming from the Milky Way galaxy.
'The sun emits low-energy cosmic ray particles that are detected here on Earth, but they are nowhere near as high energy as the particles detected at the Auger Observatory.