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.
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