“Space” is certainly an apt nickname for our cosmos, since there’s a heckuva lot of it out there.
Between here and the moon, about a quarter-million miles away, there’s virtually nothing — just stray hydrogen, helium and the odd dust particle. On far grander scales, this barrenness becomes unimaginably vast. A desolate, virtually starless, 2.5 million light-year gulf — that’s nearly 15 quintillion miles — separates our home galaxy, the Milky Way, from its nearest sizable neighbor, the Andromeda Galaxy.
Yet compared to cosmic scales, the Milky Way and Andromeda are right next door. Like neighbors awkwardly catching glances of each other through the windows, we can see Andromeda with the naked eye as a glowing smudge in its namesake constellation. The vast majority of the universe’s galaxies similarly huddle together. They gather into the equivalent of neighborhoods, cities and interconnected megalopolises known in astro-jargon as groups, clusters and filaments. Here in our Local Group, for instance, some 50-odd galaxies nestle within a dumbbell-shaped space 10 million light-years long.
In contrast to such typically close-knit galactic communities, enormous zones called voids are the boonies. For example, only several dozen small galaxies dot the Boötes Void, a spherical, bucolic region that spans a whopping 250 million light-years. (A more urban part of space might pack 10,000 galaxies into such a volume.) “Void galaxies are the loneliest galaxies,” says Kathryn Kreckel, a researcher at the Max Planck Institute for Astronomy in Heidelberg, Germany.
We’ve learned in recent decades that these void hinterlands, not galactic metropolises, are actually the cosmic norm. “Voids occupy most of the universe,” says Michael Vogeley, an astrophysicist at Drexel University in Philadelphia. “We find that over 60 percent of the universe is in voids.”
