Here's a little experiment.
Hold up your hand.
Now put it back down.
In that window of time, your hand somehow interacted with dark matter -- the mysterious stuff that comprises the vast majority of the universe. "Our best guess," according to Dan Hooper, an astronomy professor at the University of Chicago and a theoretical astrophysicist at the Fermi National Accelerator Laboratory, "is that a million particles of dark matter passed through your hand just now."
Dark matter, in other words, is not merely the stuff of black holes and deep space. It is all around us. Somehow. We're pretty sure.
But if you did the experiment -- as the audience at Hooper's talk on dark matter and other cosmic mysteries did at the Aspen Ideas Festival today -- you didn't feel those million particles. We humans have no sense of their existence, Hooper said, in part because they don't hew to the forces that regulate our movement in the world -- gravity, electromagnetism, the forces we can, in some way, feel. Dark matter, instead, is "this ghostly, elusive stuff that dominates our universe," Hooper said.
It's everywhere. And it's also, as far as human knowledge is concerned, nowhere.
And yet, despite its mysteries, we know it's out there. "All astronomers are in complete conviction that there is dark matter," said Richard Massey, the lead author of a recent study mapping the dark matter of the universe, and Hooper's co-panelist. The evidence for its existence, Hooper agreed, is "overwhelming." And yet it's evidence based on deduction: through our examinations of the observable universe, we make assumptions about the unobservable version.
Dark matter, in other words, is aptly named. A full 95 percent of the universe -- the dark matter, the stuff that both is and is not -- is effectively unknown to us. "All the science that we've ever done only ever examines five percent of the universe," Massey said. Which means that there are still mysteries to be unraveled, and dark truths to be brought to light.
And it also means, Massey pointed out, that for scientists, "the job security is great."
You might be wondering, though: given how little we know about dark matter, how is it that Hooper knew that a million particles of the stuff passed through your hand as you raised and lowered it?
"I cheated a little," Hooper admitted. He assumed a particular mass for the individual particles. "We know what the density of dark matter is on Earth from watching how the Milky Way rotates. And we know roughly how fast they're going. So you take those two bits of information, and all you need to know is how much mass each individual particle has, and then I can get the million number. And I assumed a kind of traditional guess. But it could be 10,000 higher; it could be 10,000 lower."
Which is a nice metaphor for our approach to learning more about dark matter overall. It's a lot of educated guesswork. When it comes to the bulk of the universe, "we have no idea," Massey said. "And that's what makes it exciting."