A zoomed in, computer-generated simulation of the distribution of dark matter in the universe. The top right inset shows dark-matter halos. The bottom left inset shows tiny blobs that depict dark-matter halos the size of Earth.
  • Dark matter makes up an estimated 85% of the universe. But because it doesn't interact with light, it has never been observed and is difficult to study.
  • Using a simulation of the universe, Harvard researchers found that dark matter seems to ahve a universal property: It forms halos. 
  • The findings could help future researchers detect dark matter using radiation-hunting telescopes. 
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Like a skeleton, dark matter provides the universe with structure.

The elusive stuff is thought to make up about 85% of all matter in the universe, and scientists can measure the way its gravity affects visible matter, like planets, stars, and galaxies. But it's called dark for a reason: It doesn't interact with light, making it invisible — at least to our current observation technologies.

To visualise what dark matter would look like if we could see it, researchers at the Harvard & Smithsonian Center for Astrophysics created a complex simulation that mimics the composition of the universe, including dark matter. It's the first simulation to model the universe with its dark matter from the Big Bang to the present day, according to the researchers.

The result is a visual representation of how dark matter is distributed across the universe, a pattern known as the "cosmic web." It shows how dark matter clusters together in halos connected by long filaments, as shown in the image below. Scientists think gas gets funneled along these filaments into the dense centers of the halos, where it collects, eventually forming stars and galaxies. Think of it like cars taking highways to cities. 

A zoomed in, computer-generated simulation of the distribution of dark matter in the universe, also referred to as the "cosmic web." The small, spherical blobs of dark matter scattered throughout the image are dark-matter halos.

Nobody knows for sure what dark matter is made of — which is, of course, an obstacle when simulating it. So the researchers based their model off the most commonly held theory about dark matter: that it consists of weakly interacting massive particles, or WIMPs, that are 100 times the mass of ordinary protons yet weakly charged.

In addition to a better visualization of the cosmic web, the simulation also led the scientists to discover what they think is a universal property of dark matter: It consistently forms the same types of halos.

They found that across the board, dark matter organizes itself in the same halo-like pattern. The halos all have a similar structure, no matter their size: They're densest at their centers, and become more scattered at their edges. 

"These [halos] are formed at different epochs in the universe, formed through different processes, and yet they're behaving in a predictable, universal way," Sownak Bose, a co-author of a study describing the findings that was published in the journal Nature, told Business Insider.  

The halos Bose's team simulated range in mass from Earth-sized to a quadrillion times the mass of the sun, and they surround every galaxy in the cosmos. The larger halos are estimated to weigh 10 to 100 times more than the galaxies they surround. 

Yet despite this huge size range, dark-matter halos are remarkably consistent, according to Bose.

"I could show you a picture of a galaxy cluster with a million billion times the mass of the sun, and an Earth-mass halo at a million times smaller than the sun, and you would not be able to tell which is which," he said in a press release.

The halos could emit detectable gamma rays

The new simulation sheds light, in particular, on smaller dark-matter halos, which don't surround any visible object we can detect. Whereas researchers can study large halos indirectly by looking at the galaxies they surround, hunting for smaller halos requires researchers to attempt to detect the energy released when WIMPs crash together — a process called dark-matter annihilation.

When WIMPs collide near the center of dark-matter halos, they create a burst of gamma-ray radiation. Researchers think gamma-ray telescopes should be able to detect this, but nobody has reliably detected a gamma-ray burst yet. 

"Dark matter annihilation is the only way we can identify small dark-matter halos," Bose said. 

A gamma-ray sky map produced by the Fermi Large Area Telescope.

Smaller halos formed closer to the Big Bang, when the universe was more dense, so they are at least two to three times denser than larger halos. That means their centers have more WIMP collisions, which might make them researchers' best chance of finding dark matter using gamma-ray telescopes. 

Bose said the new findings his team's simulation has yielded so far may spur new and exciting questions, like why dark-matter halos form in the first place and how they interact.

"You can ask some really fundamental and profound questions about the properties of dark matter that would not be possible to do before," Bose said.

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