Intergalactic means existing or happening between galaxies. A galaxy is an enormous collection of billions of stars, planets, and cosmic dust held together by gravity, like our own Milky Way. Intergalactic space is the vast emptiness between these galaxies, with stretches so huge that light itself takes millions of years to cross them.

Science fiction writers love intergalactic stories because they imagine civilizations so advanced they can travel between entire galaxies, not just between planets or stars within one galaxy. In reality, intergalactic travel remains purely fictional: the distances are so enormous that even our fastest spacecraft would take billions of years to reach another galaxy.

Scientists do study intergalactic phenomena, like the thin gas that drifts in intergalactic space or the gravitational forces between galaxies. When astronomers discover an intergalactic cloud or observe intergalactic matter, they're looking at material floating in that enormous void between galaxies.

You might also hear someone jokingly describe a huge distance as intergalactic, like saying your friend's new house is “practically intergalactic” from yours when they move to another state.