Why might the highest-energy cosmic rays come from outside the Milky Way Galaxy?

The highest-energy cosmic rays are believed to originate outside the Milky Way Galaxy primarily because the galactic magnetic field is not strong enough to confine these extremely energetic particles within our galaxy.1

While the exact sources remain a subject of scientific investigation, it has been suggested that these particles may be produced in active galaxies powered by supermassive black holes. Because these particles possess such immense energy, they cannot be trapped by the magnetic fields present within the Milky Way, allowing them to escape into intergalactic space.1

It is important to note that while intergalactic magnetic fields are strong enough to deflect these particles during their transit—making it difficult to trace them back to their precise points of origin—there is a physical constraint on how far they can travel. These particles undergo significant energy losses when they collide with photons of the cosmic microwave background, which may impose an upper limit on the energies of the cosmic rays we are able to detect.1