STEVE INSKEEP, HOST:
On this Halloween, let's delve into dark matter. Researchers with the large underground Xenon experiment said yesterday that their efforts to detect dark or ghost matter have not produced anything - yet. Our colleague David Greene called astrophysicist Neil deGrasse Tyson to find out what dark matter and dark energy could be.
DAVID GREENE, HOST:
This seems like an appropriate time, Halloween time, to ask you about something called the dark universe. What is it?
NEIL DEGRASSE TYSON: Well, first of all, I mean where there isn't light, it's dark, so you can...
GREENE: That seems to make sense. You've got me so far.
TYSON: There are two entities in the cosmos that both share the word dark in their name. One of them is dark matter. The other is dark energy. And the fact that we have names for them is actually a little bit misleading, 'cause that implies that we know that one is matter and one is energy. But in fact, we know nothing about either, other than that we can measure that they're there.
GREENE: So, like, you and I are matter.
TYSON: And I'd like to think we matter.
GREENE: I'd really like to think that too, but we don't know what this dark stuff is. How much of it is there in the universe? I mean, is there a lot?
TYSON: Yeah. Five-sixths of all gravity in the universe comes from an unknown source.
GREENE: That sounds like there's a lot that we don't know. So there's this five-sixths of what creates gravitational pull is actually dark and we have no concept of what exactly it is.
TYSON: Correct. And you think that's a state of ignorance. We've known the universe has been expanding. Recent measurements back in the late 1990s showed that we're not only expanding, we're accelerating. We have no idea what is causing this, but we measure it. And it is some mysterious pressure in the vacuum of space. We call that dark energy.
And that, it turns out, is the bulk of all that is driving everything in the universe. So if you add up dark matter and add up dark energy, it comes to 96 percent of what drives the known universe.
GREENE: On Earth? Is any of this stuff here? Like, I mean, I'm in a studio and I know, like, me and the microphone are probably the kind of matter that we're familiar with, but is there dark matter or dark energy kind of floating around somewhere that I don't know about?
TYSON: You don't feel the force?
GREENE: Feel the dark side.
TYSON: Feel the dark side. So dark energy manifests itself in the large scale vacuum of space, and so in little bitty places like Earth we just have no way to measure that. And dark matter, same problem. Dark matter, we see its gravity. We can't point our telescopes to it and know anything about it.
And as best as we can judge, dark matter doesn't even interact with itself 'cause you might say is there a dark matter planet somewhere around, all right? Let's go visit that. And if it is and it doesn't interact with our planet, they could pass through one another. That'd be a really cool science fiction premise.
The problem is there's no evidence for that at all.
GREENE: Okay. But you're kind of spooking me out because if there's this stuff that somehow could pass through...
TYSON: This is only kind of spooking you out?
GREENE: It's really spooking me out totally. I mean if there's this stuff that could sort of pass through us and we would never know, how is that different from the sort of supernatural ghost-spirits that we sort of talk about and make fun of?
TYSON: Oh, yeah, because the supernatural ghost spirits are in your head and no one else can verify that they're there. Whereas dark matter and dark energy, we can each devise separate experiments and agree with each other what we're measuring.
GREENE: Did George Lucas know about this research when he created the dark side in "Star Wars" or did he just get lucky and use a term that's becoming, you know, really, really popular now?
TYSON: The dark force? Well, they could tap into that for their own will, and the Jedis could. It would be interesting if somehow some of us could tap into it and use it for good or evil, right? That's the next superhero perhaps.
GREENE: That just brings it all together. There's your Halloween costume.
TYSON: Exactly.
GREENE: Thanks, as always, for joining us and Happy Halloween.
TYSON: Happy Halloween to you and all the listeners. Transcript provided by NPR, Copyright NPR.