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Scientists are still trying to educate themselves about a substance they cannot see. It's called dark matter. Physicists are certain that dark matter makes up most of the mass of the universe. They are sure that, even though they can't seem to find it. NPR's Joe Palca, the reporter of our series "Joe's Big Idea," has been asking what makes scientists certain.
JOE PALCA, BYLINE: One reason scientists are so sure dark matter has to exist is that they can see the effect it's having on the way stars and galaxies are moving. Without something providing the gravity to hold the stars in as the galaxy rotates, the stars would fly off into space, like a bunch of dishes flying off a spinning Lazy Susan. Richard Gaitskell is a physicist at Brown University.
RICHARD GAITSKELL: If you look at a typical galaxy and count up all the stars in the galaxy, which is perfectly possible for us to do this day, you find that the speed at which that galaxy rotates is simply too fast to be held together by the mass of those stars alone.
PALCA: Gaitskell says if you calculate how much gravity is needed to hold a galaxy together...
GAITSKELL: (Laughter) You find that 90 percent of the matter of that galaxy has to be some other dark material, which is why we don't see it in our telescopes.
PALCA: Once scientists were convinced dark matter had to exist, they quickly realized dark matter couldn't be something familiar, like clouds of interstellar soot. So they looked for something more exotic using giant particle accelerators and sensitive detectors and found nothing.
GAITSKELL: It cannot be a conventional particle that we already know about. That, of course, makes it so fascinating - is because it has to be new physics.
PALCA: Gaitskell says most scientists are now convinced that what makes dark matter so devilishly hard to pin down is it just doesn't interact with anything, even though there's a lot of it all around us all the time. And I mean a lot.
Gaitskell says, right now, if you stick out your hand...
GAITSKELL: (Laughter) You've probably got about 100 million dark matter particles going through your hand (laughter) every second.
PALCA: And none of them, not one, will leave any trace.
GAITSKELL: You could have your hand out for a century and still not have a single dark matter particle interact with it.
PALCA: But Gaitskell says theory predicts, at some point, one of these particles will interact with something. The question is...
GAITSKELL: Can we build an instrument that is sensitive enough to actually get the first signal?
PALCA: More than a dozen teams around the world are trying. Gaitskell leads one of the efforts. It's called LUX, the Large Underground Xenon experiment, 815 pounds of liquid xenon in a tank festooned with detectors in an abandoned gold mine in South Dakota, waiting for a dark matter particle to have one of those rare interactions. So far, nothing.
A much bigger detector is planned for later in this decade. I asked Gaitskell if it ever gets frustrating to keep coming up empty. Wouldn't he prefer to work on something with a higher chance of success? No, he said.
GAITSKELL: Even a small chance at ushering in this new generation of our understanding of both cosmology and, at the same time, fundamental particle physics - that's too good an opportunity to pass up.
PALCA: Well, you can't blame a guy for trying.
Joe Palca, NPR News.
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