New Simulation Shows Dark Matter Forms Universal Halos Surrounding Every Galaxy

Dark matter is thought to account for about 85 percent of all the matter in the universe, yet no one has ever seen it. It does not interact with light, so it is invisible to every instrument we have. What scientists can do is measure how its gravity pulls on things they can see: planets, stars, galaxies. In that sense it works like a skeleton, giving the cosmos its structure while staying out of view.

Researchers at the Harvard & Smithsonian Center for Astrophysics have now built a simulation meant to show what that hidden structure would look like. According to the team, it is the first simulation to model the universe with its dark matter all the way from the Big Bang to the present day. A study describing the work was published in the journal Nature.

The simulation produces a picture of the so-called cosmic web. Dark matter gathers into dense halos, and long filaments connect those halos to one another. Scientists believe gas is funneled along the filaments into the dense centers of the halos, where it collects and eventually forms stars and galaxies. The comparison offered is cars driving along highways toward cities.

Nobody knows what dark matter is actually made of, which makes simulating it a challenge. The researchers built their model on the most widely held theory: that it consists of weakly interacting massive particles, or WIMPs, each about 100 times the mass of a proton and only weakly charged.

The simulation also turned up what the team believes is a universal property. Across the board, dark matter organized itself into halos with the same kind of structure, densest at the center and more scattered toward the edges. The size of the halo made no difference. The ones the team simulated ranged from the mass of the Earth 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 inside them.

Sownak Bose, a co-author of the study, told Business Insider that these halos formed at different times and through different processes, and yet they behave in a predictable, universal way. In a press release he put it more vividly: show him a galaxy cluster with a million billion times the mass of the sun beside an Earth-mass halo, and you could not tell which is which.

The finding matters most for the smallest halos, which surround no visible object. Large halos can be studied indirectly through the galaxies they hold. Small ones can only be found by looking for dark-matter annihilation, the burst of gamma radiation expected when WIMPs collide near a halo's center. Bose said annihilation is the only way to identify small halos. Researchers think gamma-ray telescopes should be able to detect such bursts, but nobody has reliably done so yet.

Smaller halos formed closer to the Big Bang, when the universe was denser, so they are at least two to three times denser than larger ones. Their centers should see more WIMP collisions, which could make them the best target for gamma-ray searches. Bose said the results may also raise new questions, including why halos form at all and how they interact, questions he said could not have been asked before.