Zoomed out maps of the universe show that galaxies often cluster together in bright city-like regions. Each cosmic metropolis is connected to others by interstate highways – vast filaments of dark matter, gas, and dust, along which additional galaxies can be found. This large-scale structure is called the cosmic web.

Way out in the boondocks – far from the galaxies and filaments – are the cosmic voids. They’ve been growing larger for billions of years, emptying out as gravity pulls matter elsewhere.

This animation visualizes the early universe, when the cosmic was full of a hot plasma soup.

Cosmic voids were born when the universe looked extremely different than it does today. Instead of being speckled with stars and galaxies, the cosmos was filled with a sea of plasma (charged particles) that formed a dense, almost uniform fluid.

There were slightly denser kernels of matter, like a single ounce of cinnamon sprinkled into about 13,000 cups of cookie dough! Since the clumps had more mass, their gravity attracted additional material. Those areas grew and grew, drawing more matter together to form stars, galaxies, and galaxy clusters as the universe expanded over billions of years. Meanwhile, the spaces in between became ever emptier.

A simulation of large-scale structure forming under the influence of gravity.

Cosmic voids aren’t completely empty, though. They do have sparse galaxies, though they seem to have delayed development. Since there’s less matter, there’s weaker gravity pulling things together so stars and galaxies form more slowly. And those galaxies are isolated so they’re less likely to interact with others, which fuels growth in denser places like galaxy clusters.

But voids are mostly filled with things we can’t see. They contain a thin mist of dark matter along with a relatively larger amount of WIMPS (weakly interacting massive particles) like ghostly neutrinos than we find elsewhere in the universe. Since there’s not very much stuff in voids to create gravity, a different force reigns supreme: dark energy, the mysterious cosmic pressure that seems to be speeding up the universe’s expansion. Since cosmic voids are influenced primarily by dark energy, they offer clues about its behavior.

Astronomers haven’t thoroughly studied cosmic voids yet, but our upcoming Nancy Grace Roman Space Telescope will be wide-eyed enough to reveal those desert patches of space like we’ve never seen them before. Studying them will show how the universe is put together and how dark energy is pushing galaxies apart.

If you could fly through the cosmic web at hyperspeed, you might see a view like this simulated one!

So far, scientists have found around 1,000 cosmic voids. Roman’s 3D surveys should find tens of thousands more, both large and small, scattered throughout earlier cosmic eras than previous large sky surveys could see. That means we’ll be able to watch how the most vacant places get even emptier over billions of years. And astronomers can trace any changes in dark energy’s might by seeing how it stretches voids, where dark energy dominates, across cosmic time. 

Follow along with Roman’s journey to launch at nasa.gov/roman.