The average human body gets hit by a particle of dark matter about once a minute, according to new calculations based on several dark matter detection efforts.
Dark matter is an invisible form of material that's thought to exist because scientists have observed its apparent gravitational effects on galaxies and galaxy clusters. Scientists estimate that the mysterious substance makes up almost 80 percent of the matter in the universe.
So far no one's been able to pinpoint the particles that make up dark matter. But a leading candidate is a theoretical group known as Weakly Interacting Massive Particles, or WIMPs.
As the name implies, these hypothetical particles would have only a weak effect on regular, or baryonic, matter—they typically zip straight through most of the stuff in the universe, including people.
But WIMPs of certain masses can collide with atomic nuclei on occasion—and now it appears such collisions might happen more often than previously thought.
"Before we did this work, I thought a WIMP collided with one of your nuclei once in your lifetime," said Katherine Freese, a professor with the Michigan Center for Theoretical Physics at the University of Michigan.
"Turns out it's more likely to be one a minute."
According to theory, WIMPs were created at the dawn of the universe, just like other forms of matter.
Although WIMPs don't interact much with normal matter, if two of the particles strike each other, they annihilate—all their mass turns into energy.
"As the universe [expanded and] cooled, [WIMPs] spread so far apart they no longer annihilated and just stuck around," said study co-author Christopher Savage, a postdoctoral researcher with the Oskar Klein Centre at Stockholm University in Sweden.
Models suggest that today billions of WIMPs are streaming through Earth and its inhabitants every second.
Some experiments designed to detect WIMPs are based on the expected probability of a WIMP hitting a particular type of material—such as germanium crystals—and the amount of energy that should be released by the collision.
(Related: "Dark Matter Proof Found Over Antarctica?")
For the new research, Freese and Savage used similar calculations to look at the masses and abundances of certain kinds of WIMPs and estimate how often a particle might interact with nuclei commonly found in people.
"We know how to do these calculations, we'd just never actually done it for something like a human body," Savage said.
The results showed that oxygen and hydrogen atoms were more likely to be struck by WIMPs. And since we're made from large quantities of water, aka H2O, the human body holds lots of potential for WIMP interactions.
According to the study, some types of WIMPs smash into the nuclei of a 154-pound (70-kilogram) person ten times a year.
And for other types of WIMPs, the researchers estimate, as many as a hundred thousand particles a year likely collide with human nuclei.
Dark Matter a Danger?
Because WIMPs don't have strong effects on normal matter, collisions inside the human body shouldn't pose much danger.
But when WIMPs collide with each other, their annihilation is a much more energetic reaction.
"Say they each weigh a hundred times the mass of a proton," Freese said. "By two of them hitting, you'd get 200 times as much energy as the mass of a proton. That's pretty energetic."
If a WIMP annihilation should happen within the confines of a human body, "this could cause a mutation that would be bad for you," Freese said.
"But the odds of it happening are really low."
The dark matter-collision study was published online this month at arXiv.org and has been submitted for publication in the journal Physical Research Letters.