“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.
GROUPS: A small collection (dozens) of galaxies gravitationally bound together.
CLUSTERS: A large collection (up to thousands) of gravitationally bound galaxies.
FILAMENTS: Enormous threadlike structures made up of groups and clusters.
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,
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.”
Once thought of as uninteresting backwaters, voids are emerging as the new big thing in several realms of astrophysics. “Voids are a comparatively young
field, but people are excited,” says Bhuvnesh Jain, a professor of astrophysics at the University of Pennsylvania. Because of their profound emptiness,
voids make unique laboratories for testing why the universe looks and behaves the way it does. Astronomers can study voids to tackle the cosmological
bugaboos of dark matter and, in particular, dark energy.
“Voids are the best place to look for the signature of dark energy,” says Vogeley. If those signatures never turn up, voids might instead put the kibosh on
dark energy, ushering in new forms of gravity or even a new force of nature. If all that weren’t enough, studying those rare, loner galaxies that call
voids home should shed light on how all galaxies evolved over the universe’s eons.
Indeed, as a decade-old astronomy journal paper whimsically put it, void science is — with apologies to Shakespeare — much ado about nothing.
Spinning the Cosmic Web
The first inkling of the gaping holes in the universe’s distribution of galaxies came in the late 1970s, when astronomers began sketching out the
three-dimensional structure of the cosmos. They relied on the redshifting of galaxies’ light to estimate their respective distances: The farther away a
galaxy is, the more the expanding universe stretches its light toward the red end of the light spectrum. These redshifted galaxies’ locations outlined the
shapes of hollow pockets. The mammoth Boötes Void was discovered this way in 1981, and six years later came the boringly named Local Void, skirting our
These newfound voids upended the prevailing view of the universe as a smooth, uniform mosaic. The cosmos, we learned, is akin to Swiss cheese or foam, with
galaxies clumping by the hundreds of thousands around colossal cavities.