Galaxies are well studied, but far less is known about the vast stretches of space between them. Though it seems empty, the intergalactic medium (IGM) actually contains more matter than galaxies do – it's just hard to see because it's not shining bright as stars. Now, astronomers have used simulations to reveal new details about the structure of this matter, showing that "intergalactic pancakes" spanning millions of light-years tend to collapse into a cosmic fog.

The universe appears to be missing quite a lot of matter – the vast majority of it, in fact, is unaccounted for. Dark matter, of course, is the mysterious stuff that's hypothesized to make up roughly 80 percent of all matter, and while we can tell indirectly that it's out there, we haven't yet been able to detect it directly.

But even of the remaining 20 percent, which is made up of "normal" matter or baryons, large swathes are hard to find. It's been suggested that as little as 10 to 20 percent of baryons reside in galaxies, while the rest of them are probably swirling around in huge gas clouds between galaxies.

Because this matter doesn't shine like stars, it's hard to spot directly, but astronomers have their ways. Usually, they can tell what's there and how much by measuring the absorption of background light and other radiation like X-rays.

For the new study, a team of astronomers from Yale University, the Max Planck Institute for Astrophysics and Heidelberg Institute for Theoretical Studies has examined the physics and structure of intergalactic gas clouds using simulations. Their results have revealed a few new features about this strange stuff.

"These are flattened distributions of matter, known as 'pancakes,' that extend across many millions of light years across," says Frank van den Bosch, co-author of the study. "We found that rather than being smoothly distributed, the gas in these pancakes shatters into what resembles a 'cosmic fog' made up of tiny, discrete clouds of relatively cold and dense gas."

Previously it's been believed that these denser clouds of gas only formed close to galaxies, where the gas is already naturally denser. But this new simulation is the first to show that it can also happen in deep intergalactic space, as the cooling gas triggers an instability.

The simulation also showed that this cosmic fog is "pristine," meaning it doesn't contain heavy metals. That makes sense, since it's far enough away from galaxies that heavy metal "pollution" can't reach them.

The team says it's important to study the IGM because it feeds into galaxies and provides fuel for new stars to fire up. Knowing what's in it, and how it works, can tell us a lot about the large scale structure and composition of the universe.

"The reason galaxies are able to form stars continuously is because fresh gas flows into galaxies from the IGM," says Nir Mandelker, lead author of the study. "It is clear that galaxies would run out of gas in very short order if they didn't accrete fresh gas from the IGM."

The research was published in the Astrophysical Journal Letters. A simulation can be seen in the video below.