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A USC-led research team has created a series of supercomputer-simulated twins of our Milky Way galaxy—which could help scientists unlock new answers about one of the biggest mysteries in the universe: dark matter, the invisible substance that makes up about 85% of all matter in existence.

The research was led by cosmologist Vera Gluscevic, who is an associate professor at the USC Dornsife College of Letters, Arts, and Sciences; as well as Ethan Nadler, formerly a postdoc at USC and Carnegie Observatories who is now an assistant professor at University of California, San Diego; and Andrew Benson, a staff scientist at Carnegie Observatories.

They called their simulation project "COZMIC" —short for "Cosmological Zoom-in Simulations with Initial Conditions beyond Cold Dark Matter."

Scientists have known for decades that dark matter exists—but until now, they could not study how galaxies are born and evolve in a universe where dark and normal matter interact. COZMIC has made that possible, the team said.

The development of COZMIC and the team's results are described in a trio of studies published today (Monday, June 16) in The Astrophysical Journal, a publication of the American Astronomical Society. (See , and )

The heart of dark matter

Scientists know that dark matter is real because it affects how galaxies move and stick together. For example, galaxies spin so fast that they should fly apart, but they don't. Something invisible holds them together; many scientists believe that dark matter is at the heart of this—an idea first suggested in 1933 by a Swiss researcher, Fritz Zwicky. Research on dark matter has evolved ever since.

Dark matter is tricky to study because it doesn't emit any light or energy that can be easily detected. Scientists study dark matter by watching how it affects motions and structures like galaxies. However, that is somewhat like studying someone's shadow without being able to examine in detail the actual person who cast the shadow.

For the suite of studies, the research team took the step of deploying new physics —not just standard particle physics and relativity— and programmed a supercomputer to create very detailed cosmological simulations through COZMIC to test different ideas about what dark matter might be doing.

"We want to measure the masses and other quantum properties of these particles, and we want to measure how they interact with everything else," Gluscevic said. "With COZMIC, for the first time, we're able to simulate galaxies like our own under radically different physical laws—and test those laws against real astronomical observations."

In addition to Glusevic, Nadler and Benson, the team behind COZMIC includes Hai-Bo Yu of UC Riverside; Daneng Yang, formerly of UC Riverside and now at Purple Mountain Observatory CAS; Xiaolong Du of UCLA; and Rui An, formerly of USC.

Several dark matter scenarios

"Our simulations reveal that observations of the smallest galaxies can be used to distinguish dark matter models," said Nadler.

For the studies with COZMIC, the scientists accounted for the following dark matter behavior scenarios:

• Billiard-ball model: In this first study, every dark matter particle collides with protons early in the universe, much like billiard balls when they are first set in motion. This interaction smooths out small-scale structures and eliminates satellite galaxies in the Milky Way. The study also includes scenarios where dark matter moves at high speeds, and others in which it is composed of extremely low-mass particles.
• Mixed-sector model: This second study is a hybrid scenario in which some dark matter particles interact with normal matter, but others pass through it.
• Self-interacting model: For this third study, the scientists simulated a scenario in which dark matter interacts with itself both at the dawn of time and today, modifying galaxy formation across cosmic history.

While running these simulations, the scientists input new physics into the supercomputer to produce a galaxy whose structure bears the signatures of those interactions between normal and dark matter, said Benson.

Gluscevic added, "While many previous simulation suites have explored the effects of dark matter mass or self-interactions, until now, none have simulated dark matter interactions with normal matter. Such interactions are not exotic or implausible. They are, in fact, likely to exist."

A new day for dark matter

The team says it is a big step forward in figuring out what dark matter really is. They hope that by comparing their twin galaxies to real telescope images, they can get even closer to solving one of space's biggest mysteries.

"We're finally able to ask, 'Which version of the universe looks most like ours?'" Gluscevic said.

The COZMIC team plans to expand their work by directly testing the predictions from their simulations with telescope data so they may discover signatures of dark matter behavior in real galaxies.

This next stage could bring scientists closer than ever to understanding what dark matter is, and how it shapes the cosmos.