Major energy cosmic? — Previous ideas on how long and how far [cosmic rays] travel in interstellar space were probably incorrect. It now appears that either these nuclei are younger and have passed through less of the Milky Way galaxy than previously thought … or … cosmic radiation fills all of space, not just our galaxy. If this is so, then the physical processes of producing cosmic rays must be as common as those which produce star light. — Science News Letter, November 27, 1965
The galaxy is teeming with cosmic rays — and we now know a lot more about these subatomic particles that tear through space with tremendous energy, up to roughly 100 billion billion electron volts. The lowest-energy cosmic rays come from the sun. Zippier particles are probably shot out of supernovas within the Milky Way. The source of the most energetic (and rare) cosmic rays are an enduring mystery, but they probably originate from outside our galaxy.