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.

Our own spiral-shaped galaxy, the Milky Way, is named for its spilled-milk appearance across Earth's nighttime sky. Our two nearest neighbor galaxies--only 160,000 light-years away--are small and irregularly shaped. They were identified by Ferdinand Magellan in his captain's log during his ship's famous round-the-world voyage of 1519. Called the Large and Small Magellanic Clouds, this pair of cloudlike features is visible primarily from the Southern Hemisphere. The nearest galaxy larger than our own is 2.2 million light-years away, parked beyond the stars of the constellation Andromeda. This galaxy, historically dubbed the Great Nebula in Andromeda, is a somewhat more massive and luminous fraternal twin of the Milky Way. Notice that the name for each system lacks reference to the existence of stars. Milky Way. Magellanic Clouds. Great Nebula. All three were named before optical telescopes revealed that they were more than just distant fuzzy objects.

Without the benefit of telescopes sensitive to every part of the spectrum, we might unwittingly declare the space between the galaxies to be empty. But aided by modern detectors and modern theories, we have probed our cosmic countryside and revealed all manner of hard-to-detect things: dwarf galaxies, escaped stars, escaped stars that explode, X-ray-emitting million-degree gas, dark matter, faint blue galaxies, ubiquitous gas clouds, super-high-energy charged particles, and the mysterious vacuum energy. With a list like that, one could argue that all the fun in the universe happens between the galaxies rather than within them.