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Welcome to the Dark Side

Season 6Episode 1Nov 7, 2023

Normal matter—the kind that makes up our home planet and everything we can see—adds up to just five percent of the known universe. The other 95 percent is dark matter and dark energy, a tag team that ranks among the biggest mysteries in all of science. NASA astrophysicists Jason Rhodes and Ami Choi explain how we study this dark side and why it’s making scientists reconsider what we think we know about the universe. NASA's Curious Universe is an official NASA podcast. Discover more adventures with NASA astronauts, engineers, scientists, and other experts at

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Episode Description:

Normal matter—the kind that makes up our home planet and everything we can see—adds up to just five percent of the known universe. The other 95 percent is dark matter and dark energy, a tag team that ranks among the biggest mysteries in all of science. NASA astrophysicists Jason Rhodes and Ami Choi explain how we study this dark side and why it’s making scientists reconsider what we think we know about the universe.



[MUSIC: Still Curious? promo song – “Seasoned and Celebrated” by Matt Norman]

PRODUCER JACOB PINTER: Hey Curious Universe listeners. I’m Jacob Pinter, new producer here at the show. And I have a question for you: If you could ask a NASA scientist or astronaut anything, what would it be?


Well, here’s your chance! This season, we want to know what you’re curious about. Send us your question at, and we’ll try to track down the answer. Thanks, and enjoy the show!


[Song: Cloud Voyage by Ella Ryan]


[ARCHIVAL AUDIO: Euclid launch

Unidentified voice 1: LE, go for launch.


Unidentified voice 2: And great news, all systems are go for launch of Falcon 9 and  Euclid Space Telescope.]


Jason Rhodes: So I was very lucky in that I got to go to Florida to watch Euclid launch on July 1, 2023.


[Unidentified voice 1: T-minus 15 seconds and counting.]


Jason Rhodes: And I know exactly how close I was, because like any good physicist when the rocket launched—


[Unidentified voice 1: Three, two, one. Engine ignition. Liftoff. (sound of rockets firing)]


Jason Rhodes: I could see the intensely bright light. And I started counting seconds in my head.


HOST PADI BOYD: Jason Rhodes is a scientist at NASA’s Jet Propulsion Laboratory.  He’s an astrophysicist who studies what the universe is made of and how it’s structured. Like any physicist will tell you, on Earth, light travels faster than sound. So Jason watched the rocket ignite silently. Then he counted for about 20 seconds.


Jason Rhodes: … until I got hit with the shockwave and the sound and the rumbling.


[Sound of rocket takeoff]


Jason Rhodes: And I’m a physicist so I know the speed of sound in air, so I could figure out about how far I was away. It was about four and a half miles. And that’s probably about as close as I would have wanted to be because it was quite a rumbling and quite bright.


HOST PADI BOYD: This rocket carried a telescope called Euclid, made by the European Space Agency, with help from NASA and other scientists around the world. Euclid was headed to a point in space almost a million miles away from Earth. From there, it’s designed to make a 3-D map of the universe, including dark matter and dark energy—mysterious elements of the universe that we can detect. But we don’t know much about them.


Jason Rhodes: Euclid has its origins in part in a paper I wrote in 2004 with some colleagues. So we described this telescope in this paper. And then we started to go to space agencies and saying, Hey, we want the money to build this telescope. Now this is a long process. I, you know, pride myself on being a scientist and thinking of things sort of dispassionately and logically. And so I thought I was gonna watch this launch with that sort of sentiment. But I became very emotional. It’d been the better part of 20 years of my life and my career. But the other aspect of it for me was, I felt incredibly lucky to be part of this grand adventure that is astronomy launching a telescope into space. It’s one of, in my opinion, the proudest and most important human accomplishments is to be able to try to understand the universe that we live in.


[Theme Song: Curiosity by SYSTEM Sounds]


HOST PADI BOYD: This is NASA’s Curious Universe. Our universe is a wild and wonderful place. I’m your host Padi Boyd and in this podcast, NASA is your tour guide.


“Dark matter” and “dark energy” may sound like something Luke Skywalker had to fight off in Star Wars. Unlike The Force, they’re real. But we do notice them in galaxies far, far away. The stuff that we know here on Earth—the atoms and molecules that make up our bodies, our home planet, the moon and stars and everything else we can see—is only about five percent of the universe. The other 95 percent is dark matter and dark energy.


In this episode, let’s explore the dark side of our universe. We’ll learn why cracking the secrets of dark matter involves a battle between MACHOs and WIMPs. And we’ll hear how a couple of new space telescopes could help us solve some of these  —or give us new questions to investigate.


So let’s start at the beginning with a question that seems pretty straightforward: what is dark matter?


Ami Choi: Well, that’s a great question. I guess if we knew the exact answer to that we wouldn’t be sitting here today. Because, yeah, that problem would have been solved, and maybe I’d be studying something else.


HOST PADI BOYD: This is Ami Choi. She’s an astrophysicist based at NASA’s Goddard Space Flight Center.


Ami Choi: I study the universe on the largest scales—really just the biggest picture. I am interested in what the universe is made out of and then also the history of it and how it evolved to what we see today.


HOST PADI BOYD: We may not totally understand dark matter, but we do know there’s a lot of it.


[Song: Morning Hunt by Cesar Gimeno Lavin]


It makes up about 25 percent of the universe. But we can’t see dark matter, and it barely interacts with us here on Earth. We notice it when we look on the scale of galaxies including our own—or clusters made up of many galaxies.


Ami Choi: Dark matter is a type of matter that we indirectly observe through its gravitational effects. That’s the only way we really—that we know that it exists because it doesn’t emit light, and it doesn’t interact with light.


HOST PADI BOYD: As far back as the late 1800s, astronomers hypothesized some kind of matter they couldn’t see. In the 1930s, a Swiss astronomer named Fritz Zwicky was looking at a distant cluster of galaxies. He noticed the galaxies were moving a lot faster than he expected. The only way Zwicky could explain it, was that the galaxies had some kind of unseen mass.


Ami Choi: And so that was a clear evidence for dark matter, although at the time nobody believed him. He was considered an eccentric.


HOST PADI BOYD: Decades later, in the 1960s and 70s, an American astronomer named   confirmed that it existed. Since then, scientists have found dark matter all over the universe. 


Jason Rhodes: So we know that there’s dark matter out there because we can see how things move in relation to each other, and we know that there’s this mass that’s causing them to move in a certain way. Dark matter is not dark in the sense that it absorbs things and it appears black. It doesn’t absorb light. So “clear matter” might have been a better name for it because the light passes right through. I don’t think it has the same cachet, though, or gives the same sense of mystery.


HOST PADI BOYD: By the time Jason started his career in the 1990s, astrophysicists thought they had a pretty good handle on the main building blocks of the universe.


Jason Rhodes: When I started graduate school in 1994, we thought—the scientific community thought—that the universe was made up of matter, dark matter, and gravity. And by the time I finished graduate school five years later, there had been a real revolution in which the whole understanding of cosmology was sort of upended.


[Song: On the Brink by Paul Richard O’Brien and Theo Maximilian Goble]

HOST PADI BOYD: The culprit was dark energy. Now, dark energy and dark matter are two totally separate things. They’re not called “dark” because they’re related. They’re both “dark” because we just don’t know much about them.


Jason Rhodes: So dark energy is one of the biggest mysteries in all of science.


HOST PADI BOYD: The roots of this mystery go back about a hundred years. Back in the 1920s, astronomers were just beginning to realize that there were other galaxies out there, beyond the Milky Way.


Jason Rhodes: Well I’m sitting here in Southern California, in Los Angeles area. If I was outside, I could look up at the mountains and I could see Mount Wilson, which is the home of a very famous observatory where nearly 100 years ago an astronomer named Edwin Hubble was doing some observations.


HOST PADI BOYD: Edwin Hubble was a groundbreaking astronomer. In fact, his work inspired the Hubble Space Telescope, which has been orbiting our home planet and capturing pictures of the cosmos for more than 30 years.


Jason Rhodes: And Edwin Hubble, along with some other astronomers  at the time, found that the universe was expanding. It wasn’t static or staying the same size, as people had previously thought.


HOST PADI BOYD: Astronomers were pretty sure that in a universe full of matter and dark matter, eventually gravity would throw on the brakes. The universe may be expanding now, but all of those particles would be attracted to each other, and the expansion would have to slow down. Maybe it would even reverse, and the universe would shrink. And that’s what scientists were looking for in the 1990s.


Jason Rhodes: And so these two groups were trying to understand how much the expansion of the universe was slowing down. But instead they found a very surprising result, and that result was that the expansion of the universe is speeding up. That is, there’s something pushing the universe apart, causing it to expand faster and faster over time.


HOST PADI BOYD: Astronomers called that “something” dark energy. Today, we’ve calculated it makes up about 70 percent of the universe. But we can’t say what it is or where it comes from.


Jason Rhodes: And I like to say that “dark energy” is the name we give to our ignorance of what’s causing that accelerating expansion of the universe. So it’s sort of a catch-all term for a number of possible explanations.


HOST PADI BOYD: When astrophysicists look out into the universe, they see the effects of dark matter and dark energy everywhere. Ami says those effects are even imprinted into the shape of the universe itself. If you could zoom out and make a map of the entire universe, you’d see a cosmic web with tangles of galaxies connected by thin strands of matter.


Ami Choi: Dark matter, it has a gravitational force, and that’s an attractive force, it’s pulling things in. And then dark energy is something that makes the universe undergo an accelerated expansion, so it’s like a pulling-things-apart kind of force. And it’s really the interplay between these two forces that shape all of the universe that we can see, including all of the matter that does emit light.


HOST PADI BOYD: Even though we can’t directly see dark matter and dark energy, astronomers have come up with clever ways to detect them and even measure them.


[Song: A Closer Look by David Wittman and Marc Aaron Jacobs]


HOST PADI BOYD: One technique that Jason and Ami both use is called gravitational lensing. 


Ami Choi: And gravitational lensing is the phenomenon where light coming from very distant galaxies, on their way to us as the observer their path can be distorted by massive objects between us and where the light was emitted originally.


HOST PADI BOYD: This comes from one of the wrinkles of Albert Einstein’s general theory of relativity. Massive objects bend the fabric of space itself. Huge objects with lots of mass can warp space so much that light doesn’t travel in a straight line. It gets bent and distorted. When astronomers look at faraway galaxies, they can see dark matter bending and distorting the light headed toward Earth.


Jason Rhodes: And I can use an analogy here. Imagine you’re standing on a very calm day, in front of a very crystal clear pool, and you throw a penny to the bottom of the pool. Now, you can’t really see the water because it’s crystal clear, but you can see the penny at the bottom of the pool. And anyone who’s done this knows that you see a distorted view of that penny. It doesn’t appear as it would just on the ground. And that’s because the light from the penny is traveling through the water, and the light is bent by the water. So in this analogy, that penny is like the distant galaxy, and the water is like the dark matter. So we don’t actually see the dark matter. You don’t actually see the water. But you know it’s there. You know, the water is there because you see a distorted view of the penny. And we know the dark matter is there because we see a distorted image of these background galaxies.


HOST PADI BOYD: In fact, astronomers can measure this distortion throughout the universe by looking at millions of galaxies. With that information, they can see the effects of dark matter and dark energy. And that’s just one technique. The original discovery of dark energy came from  —the huge explosions caused by dying stars.


Jason Rhodes: Each time one of these supernova explodes, it gives off a very well-known amount of light over a very well-known amount of time. So by measuring that light, we can figure out how far away that supernova was.


HOST PADI BOYD: So imagine you and a friend are walking through the woods in the dark. It’s a still, quiet night. There’s not much of a moon, but you have flashlights to show the way. For some reason, your friend keeps running off on their own. But as long as you can see their flashlight, you know where they are. Since you know how bright the flashlight is, you can tell how far away they are. And you can see if they’re moving toward you or moving away. Supernovae give that same information to astronomers.


Jason Rhodes: And by measuring those distances and seeing how far away in time those distances are—because it takes time, billions of years for the light to reach us from these distant galaxies—we can measure the expansion history of the universe.


HOST PADI BOYD: With these techniques, scientists have gathered detailed information about our dark universe. Not bad for something they can’t see. Now, for dark matter in particular, Jason says we actually understand it pretty well in terms of how it behaves on large scales.


Rhodes: However we don’t understand dark matter at what we call the particle level. We don’t know exactly what dark matter is made up of.


[Song: Precise Timing by Paul Richard O’Brien and Theo Maximilian Goble]


HOST PADI BOYD: To explain dark matter, you might have thought about another mysterious object in the universe: black holes. After all, black holes have so much gravity that even light can’t escape them. So, they’re definitely dark . Astronomers have also wondered if black holes could explain dark matter—in other words, if they are the mass that is missing from the balance of the universe. Or maybe other objects, like a type of small, dim star called a brown dwarf.


Ami Choi: There are these different types of astronomical objects, which maybe are kind of dark in some sense. It makes it really hard for us to see them. So these are all examples of a type of objects which we call MACHOs. So, MACHOs are short for “Massive Compact Halo Object”.


HOST PADI BOYD: If MACHOs explained dark matter, that would mean it’s made of the same protons and electrons that make up everything around us. But experiments to detect MACHOs haven’t panned out. So, many scientists think that dark matter is made of something else—something a little more exotic.


Ami Choi: So we think maybe the dark matter could be a new type of elementary particle that hasn’t yet been detected but does have some of the properties that we’ve found in other ways from our astrophysical observations so that they do have some gravitational force and that they might be weakly interacting. And so the shorthand for this class of particles is WIMPs—so where WIMPs stands for “weakly interacting massive particles”. So you know, we have these WIMPs as the leading candidates. So they seem to have won out in this case over the MACHOs, which is kind of funny.

HOST PADI BOYD: In many ways, our research into dark matter and dark energy is just getting started. After all, we didn’t even know dark energy existed until 25 years ago. NASA and other space agencies have been building a new generation of space telescopes designed specifically to study the dark universe. First up is Euclid, the European Space Agency launch that Jason saw this summer.


Jason Rhodes: And the thing that’s really exciting about Euclid, is it’s got huge cameras. Lots and lots of pixels.


HOST PADI BOYD: Over a six-year mission, Euclid will show us billions of galaxies, looking back 10 billion years into the past.


Jason Rhodes: To give you some idea of how powerful Euclid is going to be, it doesn’t have as big a mirror as the Hubble Space Telescope, so it doesn’t have quite the resolution or the depth of a Hubble image. But it’s somewhat close. So every week it’s going to image as much of the sky as Hubble has in its 30 year history.


HOST PADI BOYD: All of that data will help Jason and other astronomers learn more about the expansion history of the universe. Then, they’ll be able to figure out the role played by dark matter and dark energy.


[Song: Mystery Helper by Liam Joseph Hennessy]


HOST PADI BOYD: Meanwhile, at the Goddard Space Flight Center in Maryland, NASA is building a new mission to the dark side, with help from industry and international partners.


Ami Choi: I mean, this is a biased point of view. But I’m most excited about the Nancy Grace Roman Space Telescope, since this is the project that I’m working on here at NASA.


HOST PADI BOYD: The Roman Space Telescope is still a few years away. It’s scheduled to launch by May 2027.   Like Euclid, one of the mission’s main goals is observing dark matter and dark energy. They will both make 3D maps of the universe. But the two telescopes have different strengths that can work together to create a powerful combination of observations. Euclid will take a wide survey of the universe in both infrared and visible light.


In comparison, Roman’s area of study will be narrower but much deeper. It has a lot of other science goals besides the dark universe, like finding exoplanets and studying objects in the outskirts of our solar system. And Roman’s resolution is about the same as Hubble’s, but with a field of view 100 times wider.


Ami Choi: So we have pictures from Hubble of one of our neighbors, the Andromeda Galaxy. And Hubble, it takes many, many individual pointings and then you mosaic those pointings together to get the overall picture of this nearby galaxy. And Roman can do that very efficiently because of how much area it can capture in a single snapshot. And so it can do that sort of in two passes, where Hubble it might take hundreds of passes.


HOST PADI BOYD: Once it launches, Roman will fly to the same point in space as Euclid and the James Webb Space Telescope, about one million miles away from Earth. This new fleet of telescopes has a mix of capabilities that will allow them to tag-team with Hubble and other observatories, e


Ami Choi: Hopefully they’re all flying together at the same time. And you could think of things, for example, like finding interesting objects in the Roman field of view because you’re capturing so much area at one time, and you might then go try to look at it with Hubble and JWST and get a really even deeper  view on that particular object. So they’re super complimentary in that sense.


HOST PADI BOYD: You may have seen some of the stunning images captured by the James Webb Space telescope or Hubble. Like the colorful photos of huge clouds of gas and dust where newborn stars form. For astrophysicists, these space telescopes provide something even more valuable: data to feed into equations that describe the universe.


Jason Rhodes: And what we’ve found is those equations predict new observations that we haven’t done before. And sometimes those predictions pan out and are true, and sometimes they aren’t. And then when they aren’t, and then when they aren’t, we have to revise our vision of the universe.


HOST PADI BOYD: In fact, there are already tensions in how astronomers describe the cosmos. It might be time to re-evaluate those equations… and some fundamentals we think we know.


[Song: Great and Small by Liam Joseph Hennessy]


HOST PADI BOYD: Based on what we know about dark matter and dark energy, our models of the early universe clash with what we understand about gravity. Something’s got to give.


Jason Rhodes: As we’ve taken better measurements, the problem has not gone away.


Ami Choi: Maybe there’s something that is new, that’s not accounted for in our current theories of general relativity, that means that the gravity behaves in a particular way that we haven’t currently accounted for.


Jason Rhodes: And in fact, if the problem persists after taking measurements with telescopes like Euclid and Roman, it will tell us that we either fundamentally don’t understand something about our measurements, or more interestingly, there’s new physics. Our understanding of physics is incomplete.


HOST PADI BOYD: This is why astrophysicists get out of bed in the morning. There’s always an opportunity to shatter the rules we think we know and come up with something even more interesting. Jason says it feels like that moment when he was a graduate student a few decades ago. Once again, astrophysics could get flipped on its head.


Jason Rhodes: So we live in a time where there’s these hints of these tensions. And so for me, personally, I’m really excited to see if these tensions play out. And if they do, I want to know, what’s the new physics that describes the universe?


HOST PADI BOYD: Once Roman and Euclid are both in space, it will take scientists years to analyze their data . And it’s hard not to wonder, what are we going to learn? What secrets about dark matter and dark energy are out there, just out of reach?


Ami Choi: I definitely don’t think that in my lifetime, we will have answered, you know, all of the interesting questions. None of these missions are just going to all of a sudden, give us the particular definitive evidence for understanding the universe completely. We’ll continue to learn a lot of new things and things that I couldn’t even say what it is that we’ll see because they are allowing us to explore the universe in really new ways.

HOST PADI BOYD: With all of this new information, dark matter and dark energy may not just be mysteries waiting to be solved. They could also be hints that we need to ask different questions. Every time we launch a telescope and we start looking at the universe in a new way, we learn about things that we had no idea were even out there. I think that’s the most exciting part of where we are today.


This is NASA’s Curious Universe. This episode was written and produced by Jacob Pinter. Our executive producer is Katie Konans. The Curious Universe team includes Christian Elliott, Maddie Olson, and Micheala Sosby.


Our theme song was composed by Matt Russo and Andrew Santaguida of SYSTEM Sounds. Krystofer Kim designed our cover art. Special thanks to Claire Andreoli, Barb Mattson, Amber Straughn, Liz Landau, Colin McNutt, the European Space Agency, and SpaceX.


If you liked this episode, please let us know by leaving us a review and sharing NASA’s Curious Universe with a friend.  And, remember, you can “follow” NASA’s Curious Universe in your favorite podcast app to get a notification each time we post a new episode.

Jason Rhodes: I have a two year old daughter and she’s starting to sing “Twinkle, Twinkle Little Star,” and the astrophysicist in me thinks, Well it’s not the star that’s twinkling. It’s the atmosphere that’s causing!


[MUSIC: NASA+ promo song – “What it Be” by Nwabisa Innocent Janda]

PRODUCER JACOB PINTER: Hey Curious Universe listeners, are you interested in more great stories from NASA? Our friends over at NASA TV have a big announcement: NASA is launching an on-demand streaming platform. It’s called NASA+.

You can so you can watch NASA’s Emmy award-winning live coverage, new original TV shows, and even listen to podcasts… all in one place! The best part? There’s no subscription required and it costs nothing!

You can find NASA+ on most major platforms through the NASA App on iOS and Android mobile and tablet devices… Also on streaming media players like Roku, Apple TV, and Fire TV… and online on all kinds of desktop and mobile devices. You can download the NASA app now and be one of the first to get NASA+ when it drops. Stay tuned and stay curious!