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MELISSA BLOCK, HOST:

Finally, this hour, a story that's making us feel quite small. Astronomers using telescopes in Chile have found the largest distant galaxy cluster ever seen. It's nicknamed El Gordo or The Fat One in Spanish. Just how large is it?

It's two with 15 zeros following it in solar masses.

In solar masses. That's Felipe Menanteau of Rutgers University, who led the study that discovered the cluster. And help us understand the size of this galaxy cluster. What is the meaning of that number?

FELIPE MENANTEAU: Well, it's a really, really big number, but what is actually interesting is it is the most massive cluster seen ever in the distant universe. And, you know, there's something neat about being the most massive because, you know, you win the contest right away. But it also - it's giving us an independent way of measuring the current cosmological model in terms of the amount of dark matter and dark energy that makes our universe.

BLOCK: So when you think about this discovery of the largest distant galaxy close to El Gordo that you've found, how does that play into answering some of the big questions that are still out there about the universe, the origin of the universe?

MENANTEAU: Well, one of the remaining questions is the question about the origins and where is everything is coming from, and that's something that cosmology and astronomy are trying to do. So finding El Gordo is confirming our picture of how the universe formed and what is the content in terms of dark energy and dark matter. So what we're doing is that we're actually putting a point there where at - in the very early universe where we believe the structures were formed.

BLOCK: And when we talk about a galaxy cluster, what does that mean? What's happening?

MENANTEAU: So a galaxy cluster is a bunch of galaxies together held by gravity. What I mean of a bunch, we're talking about hundreds of thousands of galaxies. And these are the largest bound structures in the universe, bound by gravity. And these are the densest places of the universe. So clusters of galaxies are interesting for two reasons. One is that we can use them as a signpost in our road for testing the growth of structure of the universe. Another reason why they're so interesting is that, as we're seeing in El Gordo, they're amazing laboratory for astrophysics.

So in El Gordo, we're also seeing emerging clusters. And not long ago, those were two independent very massive clusters that merged and we - actually, we caught them in the act of merging, of crashing against each other.

BLOCK: You caught them as they were colliding.

MENANTEAU: Exactly. We don't know exactly how long ago in terms of in astronomical - in astronomical time, it was very recent. It really looks like a comet. And what we're really seeing is that the hot gas that has been actually stripped from one cluster into the other as one actually cluster is passing through the other.

BLOCK: So El Gordo is something like seven billion light-years away. Catchy name, El Gordo.

(SOUNDBITE OF LAUGHTER)

BLOCK: How did that - did you come up with that?

MENANTEAU: Well, we knew that we have found something unique and extraordinary. I mean, there might be less than a handful of clusters like this in the whole universe, and we wanted to give it a name so people will remember it because in human life, being massive is not something that you're proud of. You're actually always trying to go in the other direction. So we decided to call it El Gordo in order to honor the Chilean connection. And, you know, we were able to get away with calling somebody fat without nobody being offended.

(SOUNDBITE OF LAUGHTER)

BLOCK: Nobody would - nobody will mind seven billion light-years away.

MENANTEAU: I hope so. And, you know, yeah, yeah, we're not going to hear the shouts from here.

BLOCK: I've been talking with astrophysicist Felipe Menanteau of Rutgers University. He led the study that discovered El Gordo, which is the largest distant galaxy cluster ever seen. Thank you so much.

MENANTEAU: Thank you. It was my pleasure.

(SOUNDBITE OF SONG, "GALAXY SONG")

MONTY PYTHON: (Singing) Our galaxy itself contains 100 billion stars. It's 100,000 light-years side to side. It bulges in the middle, 16,000 light-years thick, but out by us, it's just 3,000 light-years wide. We're 30,000 light-years from galactic central point. We go round every 200 million years. And our galaxy is only one of millions of billions in this amazing and expanding universe.

AUDIE CORNISH, HOST:

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