Does he let people drink in the observatory?

Right, we're breaking the rules.

I'm Marnie Chesterton, host of CrowdScience, and for a previous episode on

the BBC World Service I climbed up to a little-known observatory in central

London, partly to drink hot cocoa but mainly to uncover the secrets of the Universe.

If you look up, you can see it's sort of hazy. There are stars, you can see

the stars but it's sort of hazy. That's just pollution in London and it

means that you can see bright things. But it's quite difficult spotting galaxies.

Step on the planks.

The reason the CrowdScience team tackled

ice-covered planks at nighttime was that listener Koon-Hou from Singapore

sent us a question and we exist to answer your science queries. Here's a quick recap.

My question is regarding dark matter and if there's any way that we

can understand it more and maybe find it useful for daily living.

Our quest to find Koon-Hou answers took us down the deepest mine in the UK.

How deep is it?

It's 1.1 kilometres.

So how long do we have to spend in the lift?

Seven, eight minutes.

That's a long time to spend in the lift.

And we met professor Katherine Freese from the University of Michigan.

And I'm also a guest professor at Stockholm University.

And how long have you been working on finding dark matter?

I have been working on it since I was a

graduate student so I'm gonna say that was 25 or 30 years ago. Oh more than that.

OK, so don't tell people.

We asked Katherine what real-world applications

might come from this search for dark matter.

You know at this point I don't

know what those are gonna be but the past history shows that there will be

something really major that comes out of it.

So I can give you some examples from the past.

One of the big ones that came out

of nuclear physics is MRI. So nowadays MRI is used in hospitals everywhere

where - my shoulder hurts, what's exactly going on? -

so they did an MRI of that.

But that again was a surprising off-shoot. It was based on detectors for

nuclear physics. Then they turned out to really change our lives for the point of

view of health.

You can hear more on this if you go back and listen to that show.

Just search for 'why does dark matter matter?' in your podcast app.

So that was then.

Now CrowdScience often turns into a kind of conversation with our

listeners because episodes spawn more questions and it turns out that you lot

love a physics mystery. We received loads more emails with questions relating to

dark matter. Questions that we're going to tackle today.

To help me I'm joined in the studio by two astrophysicists. Down-the-line from

the University of Arizona is Burçin Mutlu-Pakdil and sitting next to me is old

friend of the show Matt Middleton from the University of Southampton.

Welcome to you both.

Thank you.

Old?

Old as in been on the show before.

Grizzled.

Yes.

First I think we need a

recap on what dark matter is. Burçin, you first.

So, most of the things that we

see and we are familiar with is normal matter and it is only 5% of the Universe.

So 25% of the Universe is dark matter. That's the mysterious object. We don't

know much about the nature. We cannot touch it, we cannot smell it, we cannot hear it.

But it is there because we can see its gravity there and the rest of

the Universe, the giant portion, is dark energy which is much more mysterious

than dark matter. We just know it is energy that makes the Universe expand.

OK so this stuff, dark energy and dark matter, it's most of the make-up of the Universe

and dark matter is five times as much as all the stuff that we can see -

the stars and you and me and everything.

Everything that I'd consider to be stuff.

How do we see normal matter like the

microphone in front of me and you?

We see objects because of light, which is

electromagnetic radiation and we see objects because the light bounces off to

our eye and from that we can see the objects. But the dark matter doesn't have

that effect. It is just transparent. It just goes through it. So because of that

we cannot see them.

So if you shone a light on dark matter, what would happen?

It'd go through.

What we call the interaction, the ability for this dark

matter to interact with anything is incredibly small except we think via gravity.

So let's talk about gravity.

Gravity is the attraction between two things that have mass. The reason we know that

there's dark matter out there is because matter has mass. The bigger the mass, the

stronger the gravitational pull. Our planet orbits the Sun because the Sun's

massive mass pulls it into orbit. So we can't see dark matter. But can we see gravity?

Yes and a Swiss astronomer first noticed this whilst watching how

galaxies move. His name was Fritz Zwicky

This is a very famous scientist called Fritz Zwicky and he looked at galaxy clusters. Now galaxy

clusters contain thousands and thousands of galaxies and we think that these

things should be essentially not flying apart. So if you look at how fast they're

going you can work out how much mass there should be there and Fritz Zwicky

discovered that in order to keep them shuffling around all over the place and

not flying apart yet to have much more mass there than you could see in the stars.

I wanted to see this for myself and so paid a visit to professor Malcolm Fairbairn.

He's the one with a telescope dome on the roof of his university,

King's College London.

If you look up there you'll see a cloud in the middle of the eyepiece and it

really doesn't look like very much. In fact you might find it quite difficult to see it.

No, I can see it. Yeah there's a fuzzy bit.

Mm-hmm. Yeah so that's the very centre of the Andromeda galaxy.

Is Andromeda our closest?

Andromeda's is the closest big galaxy.

There's a few little ones that are closer. Well there's lots of little,

very little ones, that are closer but Andromeda's about the same size of the

Milky Way and Andromeda is moving towards us. I think it's about 150

kilometers per second and we can use that observation to weigh the entire

system and the answer is that the whole thing weighs something like five million

million times the mass of the Sun and we can't see anywhere near that number of

stars so there has to be something else there and we think that most of it is dark matter.

So there's a whole load of stuff out there that's stuff that we can't see

but we think is there?

Yeah and there's lots and lots of other observations

which suggests the same thing. From the smallest galaxies right up to the

largest clusters, we see that on every scale we need something to explain how

quickly things are moving around that we can't see.

Physicists like Malcolm have

calculated the amount of mass in things like the Andromeda galaxy and there's lots

supposedly there that we can't see, hence dark matter.

Burçin, can we talk about what dark matter might be?

There's a huge discussion about that,

we really cannot say anything about its nature yet so we right now we

have a promising hypothesis that suggests that it is called matter which

doesn't refer to the temperature but it is referring to the speed of that

particle which means it is moving very slowly and based on these cold dark

matter idea right now we could actually explain most of the observations that we have today.

Cold meaning slow moving. Dark meaning can't see it.

Matter meaning stuff with mass. The cold dark

matter hypothesis fits what we can see but physically I mean what is it?

This is where theoretical physicists come up with ideas that explain what we can observe.

There are several competing theories with great sounding names like WIMP.

Well it turns out they're WIMPS. Which I don't mean that they've got no

sort of spine or you know any fortitude. What I mean is they're weakly-interacting

massive particles - WIMPS - and our best guess of the minute is that

they formed in the very, very early Universe. Now our Universe went through

something called the Big Bang. OK now the Big Bang was an explosion in space

and time, of space and time, and it was really, really hot. The great thing about

things that are really, really hot and you've got really high-energy radiation is that

you can spontaneously create matter out of radiation. The remnants of those

particles are what we think is dark matter.

While Matt is busy thinking about what dark matter is, listener Gautam is

thinking about what this mysterious stuff might contain.

Hello, I'm Gautam from New Delhi in India. My question for CrowdScience is this.

If there is dark matter and dark energy does it mean that there could be dark life too?

So since I was you know going through scientific stuff and I realised that what you need

for life is energy and matter. So now if there is dark energy and dark matter it

was one of those thought experiments so since I couldn't find an answer who

better than to ask outside?

Who better indeed and it kind of makes sense

because the stuff that we're made up of is only 4% of the Universe and if the

rest of the Universe is dark, what's to say that there isn't something or

someone somehow lurking out there?

Exactly, so if there is such a huge chunk

that is absolutely unknown, why discount anything?

Do you imagine it as something

that is around us or is it in a galaxy far, far away?

It has to be around us like

what we are given to explain is dark matter is everywhere, it's in this room,

it's outside the rooms, it's everywhere. So if we are surrounded by it, we are living in it.

Just listening to what you guys are talking about I was imagining something

like a massive jungle all around us with, like, I don't know, dark lizards.

Exactly, yeah.

I don't know what else... flying all around us and yet we're totally unaware.

Totally, yeah, exactly.

It's kind of nuts isn't it?

That was producer Graihagh and Gautam and me going through his thought experiment,