Astronomy Guide

Between the Galaxies
In the grand tally of cosmic constituents, galaxies are what typically get counted. Last time I checked, the universe contained a hundred billion of them. Bright and beautiful and packed with stars, galaxies decorate the void of space like cities across the landscape at night.

But just how voidy is the void of space? (How empty is the countryside between cities?) The in-your-face splendor of galaxies may persuade you that nothing else matters. But the universe is teeming with hard-to-detect things that may be more interesting, or more important to the evolution of the universe, than the galaxies themselves.