https://phys.org/news/2018-07-century-old-riddle-resolveda-blazar-source.html

Since they were first detected over one hundred years ago, cosmic rays—highly energetic particles that continuously rain down on Earth from space—have posed an enduring mystery: What creates and launches these particles across such vast distances? Where do they come from?

Because cosmic rays are charged particles, their paths cannot be traced directly back to their sources due to the powerful magnetic fields that fill space and warp their trajectories. But the powerful cosmic accelerators that produce them will also produce neutrinos. Neutrinos are uncharged particles, unaffected by even the most powerful magnetic field. Because they rarely interact with matter and have almost no mass—hence their sobriquet "ghost particle"—neutrinos travel nearly undisturbed from their accelerators, giving scientists an almost direct pointer to their source.