Emptiness wins. A galaxy crammed with hundreds of billions of stars still offers a spacecraft more radiation risk than impact risk because almost all that mass is locked in tiny islands. Between those islands, the interstellar medium is so thin that in a cubic meter you often find fewer atoms than in the best laboratory vacuum, turning physical collisions into statistical rarities rather than routine hazards.
The harsher truth is that gravity is a miser. It pulls gas and dust into dense stars and planets, but only across a minute fraction of the total galactic volume, leaving stretches of tens of trillions of kilometers with almost nothing solid inside. That geometry means an object can cross vast orbits without ever grazing a rock, while charged particles and high‑energy photons from supernova remnants and active stellar coronae sweep straight through, limited mainly by magnetic fields and cross‑sections, not by walls or surfaces.
Most misleading of all is the bright band of a spiral disk. It looks crowded. It is not. Average stellar separations run to light‑years, and the cross‑section of a star or planet is tiny compared with those gaps, so collision probabilities stay microscopic. Radiation, by contrast, propagates as inverse‑square flux from many distant sources at once, so a solitary probe is statistically far more likely to be pierced by a storm of particles than to ever strike anything solid.
