Scientists Learned a Surprising Thing About Dark Matter's Shape

It’s strange to think about how much research has gone into characterizing dark matter when we’ve yet to actually observe the stuff.

But scientists actually know plenty about dark matter: It makes up nearly 85 percent of the total mass in the universe, and it interacts with normal matter only through gravity. We’ve never fully been able to pinpoint what it’s exactly like in a physical sense, however.

Now, researchers from the University of Washington think we have a concrete idea. Dark matter is not fuzzy, as many people have recently hypothesized, but rather it adheres to the “cold dark matter” model.

In findings published in the July 20 issue of Physical Review Letters, the research group outlines the results of a set of simulations focused around characterizing dark matter and determining whether the fuzzy model or the cold model is more probable.

“For decades, theoretical physicists have tried to understand the properties of the particles and forces that must make up dark matter,” Vid Iršič, a postdoctoral researcher at the University of Washington and lead author of the paper, said in a statement. “What we have done is place constraints on what dark matter could be — and ‘fuzzy dark matter,’ if it were to make up all of dark matter, is not consistent with our data.”

The fuzzy model — the more recently trending of the two — posits that dark matter is comprised of an ultralight particle whose movement is best explained by principles related to quantum mechanics. The cold model is older, more conventionally accepted, and suggests dark matter is made of massive, slow moving particles that weakly interact with their surroundings. The cold model is better for explaining why dark matter clumps up into globular masses, as observations of different objects’ movements suggests it does.

Each has its pros and cons, but they can’t both be correct — at least not in absolute terms.

Iršič and his colleagues decided to run simulations based off of new data related to what’s known as the intergalactic medium (IGM), where dark matter is located throughout the cosmos. Hydrogen in the IGM absorbs light emitted by stars, quasars, and other objects, and this scatter helps scientists study space.

The research team looked at IGM interactions with light emitted by quasars, which are intensely active celestial objects with supermassive black holes at the center. That light data was plugged into a supercomputer along with other numbers related to dark matter particles. Simulations were run based on those numbers, and the results were predictions that better support cold dark matter over fuzzy dark matter.

“The mass of this particle has to be larger than what people had originally expected, based on the fuzzy dark matter solutions for issues surrounding our galaxy and others,” said Iršič.

Still, the study cannot answer other issues relevant to dark matter research, nor does it outright nix the idea that an ultralight “fuzzy” dark matter particle exists. More research, as always, will be needed to really shed light on the dark stuff.