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IRA FLATOW, host:

This is TALK OF THE NATION: SCIENCE FRIDAY. I'm Ira Flatow.

Like travelers jostling one another in a busy bus terminal, colliding galaxies have given us our first direct evidence of the mysterious dark matter.

Dark matter and dark energy make up about 95 percent of our universe. But being dark, it's hard to see them; we've only felt their presence.

But now, observing a collision of galaxies located over three billion light years away, astronomers have found direct proof that dark matter does exist. Why does this matter matter?

Sean Carroll is a cosmologist and senior research associate at California Institute of Technology in Pasadena. He joins us by phone. Welcome back to SCIENCE FRIDAY, Dr. Carroll.

Dr. SEAN CARROLL (Senior Research Associate, California Institute of Technology): Thank you, Ira.

FLATOW: Do we know what the dark matter is?

Dr. CARROLL: No, we don't. We know some things about it. We know some of its properties and we know more or less where it is, but it's some particle that we don't know much about. It's not anything that has ever been produced in any experiment ever done in human history.

FLATOW: So then how do we even know it exists?

Dr. CARROLL: Well, Einstein told us a long time ago that everything in the universe has a gravitational field. Everything creates gravity around it. So no matter how invisible something is, no matter how reluctant it is to interact with us, it will lead to gravity, and with the dark matter collects in galaxies and clusters of galaxies. And then we can weigh those galaxies and clusters by looking at how fast things are moving within them. And from that, we deduce that there's a lot more matter there than the visible matter, the ordinary matter, and so it must be dark matter.

FLATOW: And let's not confuse them - the dark matter with the even more mysterious dark energy.

Dr. CARROLL: That's right. Dark energy is a brand new thing in the sense that it was first discovered in 1998 - not that long ago. And dark matter is actually relatively normal compared to dark energy.

Dark matter is some particle that we haven't detected yet, but we know what particles do. We know how they move around.

Dark energy is inherent in empty space. Everywhere in the universe, every cubic centimeter has the same amount of dark energy as far as we can tell. It doesn't go away as the universe expands. It doesn't collect from here and there. But it makes the universe accelerate, which is why we know that it's there.

FLATOW: Let's talk about what you actually did - how you were able to detect or see the dark matter that you could not do before. What happened here?

Dr. CARROLL: Sure. Well, I should first say that I didn't do it. Some astronomers working with NASA satellites and also ground-based observatories did it. But what they did was they observed a case where nature was nice enough to move the dark matter away from where the ordinary matter was.

We've known for years and years that something was going on in galaxies and clusters. The gravitational fields were too strong to be explained by just the ordinary matter. But it could be dark matter or it could be some modification of gravity. Maybe gravity is just stronger than we expect on cosmological scales. And the reason why it was hard to tell the difference is because everywhere there was dark matter there was also ordinary matter there.

What you would like is to take a big cluster of galaxies - with a lot of dark matter and ordinary matter - and sweep out all the ordinary matter, leaving only the dark stuff. And, in fact, nature did this for us because it had two clusters of galaxies that collided. They went right through each other.

The ordinary matter ran into the ordinary matter from the other cluster, heated up and got stuck in the middle. But the dark matter doesn't interact. The dark matter just went right through.

So when scientists took two different pictures of this cluster, one looking at X-rays to determine where the normal matter was, and the other inferring where the dark matter is from gravitational lensing, which we can talk about, they noticed that the dark matter was in a separate place. There were two big blobs of dark matter on either side, a whole bunch of ordinary matter in between.

It's just not possible to explain that using a modification of gravity, so this makes us quite confident that there really is dark matter there.

FLATOW: So you actually didn't - you were able to see the separation of the two kinds of matter, but you still could not see the dark matter being dark?

Dr. CARROLL: No, that's right. You can only see the influence of the dark matter through its gravitational field. But the point is that now we have a gravitational field that is pointing in the direction where there's not that much matter. So the way to explain that is that there is some dark stuff there, stuff that we don't actually see.

So the point is that the next thing to do is to figure out what that particle is that is the dark matter, and we're trying to do that with experiments here on Earth.

FLATOW: What kind of experiments?

Dr. CARROLL: Well, the best thing would be to detect it directly, so people are building underground laboratories, shielding themselves from cosmic rays and other forms of interference, and hoping that the dark matter is interacting with ordinary matter but just really, really weakly.

And if we are very sensitive and very careful about looking for it, we'll see a dark matter particle bump into a detector and find it directly that way.

The other thing to do is to build a particle accelerator to collide very energetic particles with each other and see what new particles they make in those collisions. And we have a big particle accelerator with whom that we're putting the last touches on it at CERN in Geneva, and it's going to turn on next year.

We're very hopeful that among the many things we discover at this new particle accelerator will be a good candidate for dark matter.

FLATOW: So you have to find new particles in nature then that we don't know about?

Dr. CARROLL: Absolutely. That's what makes it so exciting, that all the particles in nature that we know about, that we've made in the lab, are not good candidates for what this dark matter is. It has to be some new kind of particle.

And theorists have been very energetic in coming up with different models for what it could be, so we have plenty of possibilities. But the possibilities are all very different from each other, so we would love to pin down which one is right.

FLATOW: Mm-hmm. Sean Carroll, thank you for taking time to talk with us.

Dr. CARROLL: Sure. Thank you, Ira.

FLATOW: Sean Carroll is a cosmologist at Caltech.

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