ACLogoDate:  January 4, 2009


Title:  Who Ordered the Dark Matter and Dark Energy?

Podcasters:  The Astronomy Cast team of Dr. Pamela Gay and Fraser Cain

Description: Fraser Cain and Pamela Gay are the hosts of Astronomy Cast, a weekly astronomy podcast that follows a facts-based journey through the Universe. One of the common complaints they get from listeners is that they just don’t like “dark” stuff. You know, dark matter and dark energy. Why do astronomers have to go and invent something called dark matter or dark energy? In this episode, Fraser and Pamela provide a useful analogy that will help astronomy enthusiasts understand what dark matter and dark energy are and why evidence demands these placeholders for a series of unexplained phenomena. Their analogy begins with an all too familiar experience: a well worn car starting to experience a mysterious mechanical problem.

Organization:  Astronomy Cast Podcast 

Today’s SponsorAstronomy Cast  


Fraser Cain: This is your 365 Days of Astronomy for January 4, 2009.  This is our first 365 Days of Astronomy and there should be many more.  So we hope you enjoy it.  Our contribution to 365 Days of Astronomy this month is based on a regular complaint that we get at AstronomyCast and here’s a quote: “I don’t like all of this DARK stuff, Dark Matter, Dark Energy.  Why do Astronomers have to go and invent something new?  Is this just a way to get more funding?”

Now for those of you who share this concern, we want to take a moment of your time and give you a helpful analogy, a way to help you understand that Dark Matter and Dark Energy are just placeholders – a way to collect mounting evidence for very real phenomena.  This analogy begins with…

Pamela: Your car.

Fraser:  Your car. [Laughter] Okay so then give us the car analogy.

Pamela: So, you’re driving down the street.  You’re in your car.  Life’s going well and you hang a left and all of a sudden a funny noise starts happening.  But then it goes away and you keep going – no big deal.  But the next time you turn left, you get a funny noise again.  So, you’re not sure what it is.  You’re not an auto mechanic – or at least I’m not an auto mechanic – and all you know is there’s something going on.  Every time you make a left-hand turn, it makes a funny noise.

You have a piece of evidence, you have a problem, and you don’t know what it is.  So you give it a name.  You might name it the left-hand turn problem.  Well, with Astronomy, we had this problem that started out with we looked at things in the outer edges of our Galaxy and they were moving too fast.

Then we looked at things at the outer edges of OTHER Galaxies and they were moving too fast.  So we decided there must be stuff we can’t see because that would be a good cause and we called it Dark Matter.  Well, with your left-hand turn problem, you might say it’s a dark turning is the problem.

Fraser:  [Laughter] Dark turning.

Pamela: You don’t know what it is so you give it a name.  You might notice as you’re driving down the street, if you turn really slowly there’s no noise.  If you turn really fast, it sounds like your car is going to explode.  So now you have more evidence.

Fraser:  Right and this is where you might go to your mechanic and say:  “Here’s the problem, noise turning left; go slow – no noise; go really fast and I’m worried my car is going to explode.”

The mechanic can start saying it might be (we’re not mechanics [Laughter]) this or that or something.  “We can try and take your car apart, but why don’t you go out, drive some more, gather some more data, and maybe we can figure out what it is.”

Pamela: The Universe has been really nice and friendly and has started to provide us more and more data as we found more and more ways to observe it.  We’ve been able to discover that whatever this crazy Dark Matter stuff is, it bends light that is passing through it the same way normal Matter does.

By looking at this bending of the light, this gravitational lensing, we’re able to map out where this completely invisible stuff is located.  This tells us it’s STUFF.  Now we’re starting to build up more and more clues.  It’s something that interacts with Gravity.  It’s something that doesn’t produce light.  It’s something that bends light.  This is kinda cool and we now know it is stuff.

We don’t know what the stuff is so we still have this placeholder name.  But we have more and more evidence.  We know it’s not some missing Force.  We know it’s not some misunderstanding of Gravity at least at the largest scales.  We know it’s actually just STUFF.  And that’s cool.  That stuff we didn’t know about 10 or 15 years ago.

Fraser:  Right and so from a data collection standpoint now, Astronomers have gathered really detailed maps of the distribution of Dark Matter around us in all directions in the Universe.

They do this by seeing how it bends light. They can do it with such a degree of accuracy now that they can see how it clumps. They can see how it’s strung out into long lines and how it seems to form a scaffold that the Galaxies in the clusters of the Galaxies in the Universe seem to be stuck inside of.

Pamela: And we see it all different scales.  We see it both at small scales of – let’s look at a Galaxy Cluster which doesn’t seem that small but when you’re looking at the whole Universe it’s kind of small – we see it at the scale of there’s a cluster called the Bullet Cluster where we can see the distribution of Dark Matter just around this one cluster of Galaxies.

Then we can see it across the entire Universe. This is the stuff that the Galaxy Clusters themselves that the large-scale structure of the Universe is following.  But not everywhere that there is Dark Matter is there Luminous Matter and that’s kinda cool to know.

Fraser:  So the same goes with Dark Energy.  Same thing, we’ve got back in 1998 Astronomers were measuring the distance to a distant Super Nova and realized that they weren’t where they were supposed to be.  Same thing – this is your you know, left-handed turning car noise problem.

Pamela: [Laughter] Except in this case it’s more like some sort of a weird problem with your speed indicator where whenever you’re going 70 it says you’re really going 60 but when you’re going 20 it agrees with you that you’re going 20. Here we have this even weirder problem to try and diagnose.

Fraser:  And so the same thing.  Astronomers don’t know and they’re okay to say we don’t know.  Then they just say let’s just call it Dark Energy and I guess it’s just based on Dark Matter [Laughter] we don’t know what it is.

We’re just going to sort of collect all of the data that comes in, all of the additional questions that pop up, all of the constraints.  You know, it’s moving this fast but it’s not moving that fast.  They put that all into this collection and just say this is everything that we know so far.

We don’t know what it is.  We don’t know what causes it.  We don’t know where it came from.  We don’t know but for what we do know, we’ll collect it together until we do know more.

Pamela: We are slowly getting a building body of evidence.  Back in 1998 when it was discovered, all we really knew was there is this mysterious something.  We can’t even say is it a Force, a Pressure is it an Energy, a Field, what is it?  There’s just this mysterious something that was causing the Universe to accelerate itself apart.

Well since then we’ve been able to figure out that while it seems to be having the same effect per volume of Space across all of Time and all of Space, this means that if you take any cubic meter of the entire Universe at any time it’s going to be experiencing the same mysterious something trying to push it apart.  That’s kind of cool.

We’re developing this understanding of we know that in the past it was still there as far as we know but it wasn’t the dominant effect on the Universe whereas it is today.  As we build these new pieces of evidence it starts to give us an idea that it’s a Pressure or a Force.  But we’re still kind of out in the dark.

Fraser:  So for those of you who don’t like it, then the onus is on you to disprove the data that has been collected so far, to say no, there isn’t Mass out there that is bending the light with its Gravity.

Or, no the Super Nova are exactly where they’re supposed to be and they’re not moved further than the expansion of the Universe should indicate.  Disprove that data.  But right now there is so much confirmation out there that it’s a very difficult thing to do.

Pamela: So we’d encourage all of you to take a facts-based journey and follow to see where the facts can take you.  If you want to learn more, Fraser and I have a lot more for you to listen to over on AstronomyCast.

Fraser:  Perfect.  And you can find that at and we go into great detail about both Dark Matter and Dark Energy in two separate episodes.  We hope you get a chance to listen to that.  Thanks Pamela.

This transcript is not an exact match to the audio file.  It has been edited for clarity.  Transcription and editing by Cindy Leonard.

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.