Dark Matter: Experiment to shed light on dark particles


BBC World Service science reporter Rebecca Morelle has been inside the Darkside50 experiment

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In a man-made cavern, deep beneath a mountain, scientists are hoping to shed light on one of the most mysterious substances in our Universe - dark matter.

The Gran Sasso National Laboratory seems more like a Bond villain's lair than a hub for world class physics.

It's buried under the highest peak of Italy's Gran Sasso mountain range; the entrance concealed behind a colossal steel door found halfway along a tunnel that cuts through the mountain.

Start Quote

The feeling is that dark matter could be just around the corner, so everybody is rushing to be the first to find it”

End Quote Stefano Ragazzi Director, Gran Sasso, National Lab

But there's a good reason for its subterranean location. The 1,400m of rock above means that it is shielded from the cosmic rays that constantly bombard the surface of our planet.

It provides scientists with the "silence" they need to understand some of the strangest phenomena known to physics.

Inside three vast halls, a raft of experiments are running - but with their latest addition, DarkSide50, scientists are setting their sights on dark matter.

Everything we know and can see in the Universe only makes up about 4% of the stuff that is out there.

The rest, scientists believe, comes in two enigmatic forms.

What is dark matter?

dark matter superstructure
  • Normal matter gives out or absorbs light to make it visible, but matter doesn't have to interact with light this way
  • Astrophysicists calculate that there isn't enough visible matter to explain the rotation of galaxies
  • They proposed a type of matter that we can't detect in the normal way - dark matter
  • You can't see dark matter directly with telescopes, but its gravitational effect can be seen on visible matter
  • Dark matter should be all around us, so scientists are developing new ways to detect these mysterious particles

They predict that about 73% of the Universe is made up of dark energy - a pervasive energy field that acts as a sort of anti-gravity to stop the Universe from contracting back in on itself.

The other 23%, researchers believe, comes in the form of dark matter. The challenge is that until now nobody has seen it.

Dr Chamkaur Ghag, a particle physicist from University College London, explains: "We think it is in the form of a particle.

"We have protons, neutrons and electrons and all these regular normal particles that you associate building things with. We think dark matter is a particle too, it's just an odd form of matter in the fact that we don't perceive it very readily.

"And that is because it doesn't feel the electromagnetic force - light doesn't bounce off it, we don't interact with it very strongly."

Physicists have called these dark matter contenders Weakly Interacting Massive Particles - or WIMPS.

They believe millions of them are passing through us every second without a trace.

But very occasionally one will bump into a piece of "regular" matter - and that is what they are hoping to detect with DarkSide50.

Gran Sasso Scientists hope to detect that rare event when a particle of dark "stuff" bumps into regular matter

Inside a house-sized tank, a large metal sphere holds a particle detector called a scintillator.

This container is filled with 50kg of liquid argon and a thick layer of the element in its gas form.

"If a dark matter particle comes in and hits the argon, the recoiling atom gets a kick of energy and it quickly tries to get rid of it," says Dr Ghag.

"In argon it gets rid of it by kicking out light; it sheds photons.

"But it also gives charge: some electrons that are liberated from the interaction site. And those electrons drift up into a gas layer, and when they hit the gas and you get another flash of light."

Dark matter detector As dark matter particles steam through the detector, scientists hope that a few will collide with the argon atoms. This will generate two flashes of light - one in the liquid argon and another in the gas - which will be detected by the receptors.

Until now, the hunt for dark matter has proved elusive.

Some experiments claim to have seen signals of dark matter in the form of annual modulation.

This is the idea that the number of these particles we should be detecting changes as the seasons change.

That's because as the Earth moves around the Sun, it is moving into a stationary field of dark matter - and for half the year it will be moving against the tide of dark matter - just like driving into the rain. But for the other half it will be moving with this tide and less dark matter will hit.

But other researchers have questioned attributing these seasonal variations that have been detected to dark matter.

Cavern The experiments are housed in a cavernous room underground

Other experiments have run for long periods of times without so much as a hint of the stuff.

One, called XENON100, which is also in Gran Sasso, ran for the course of a year, but only saw two "events" - not enough to rule out that this might have been some stray background radiation.

But with DarkSide50, there seems to be a new push to find some answers.

Alongside this experiment, another large detector - LUX - which is in a gold mine in South Dakota in the US will soon be coming online.

And in the next few years, scientists are planning even more ambitions detectors, such as XENON1T and LUX-Zeplin - they are hoping to find the first experimental evidence of these particles.

Aldo Ianni, from the DarkSide50 team, says: "Dark matter is really a major scientific goal at the present time.

"It will help us understand a big fraction of our Universe that we don't understand at the present time. We know there is dark matter - but we have to understand what this dark matter is made of."

Fruitless search?

Professor Stefano Ragazzi, director of the Gran Sasso National Laboratory, hopes that the first glimpse of dark matter will be in his research facility.

"There is competition amongst different experiments - so when you compete you prefer to win rather than coming second or third," he explains.

"The feeling is that dark matter could be just around the corner, so everybody is rushing to be the first to find it."

However, he admits that there is always the chance that these experiments may find nothing at all - and dark matter may not be in the form of WIMPs.

Gran Sasso sign The director of Gran Sasso hopes the first glimpse of dark matter will be made at his facility

Professor Ragazzi says: "We may find that we have the wrong hypothesis… [dark matter] may be something completely different.

"But it may be even more interesting not finding dark matter than finding it."

In the next few weeks DarkSide50 will be fully kitted out, the surrounding tank flooded with purified water, and then the scientists will have to watch and wait.

But Dr Ghag says despite the uncertainties, the potential reward of finding dark matter would be huge.

"If we did find dark matter, then we'd have done would be to solved one of nature's best kept secrets. And that would have been to have figured out what a quarter of the Universe is made of," he explains.

"That would be a revolutionary discovery - it would change our understanding of the Universe, the way it formed, and the way it will evolve."


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  • Comment number 402.

    All this user's posts have been removed.Why?

  • rate this

    Comment number 401.

    @399 Dear John,

    I am intrigued - did you mean that I should stop taking the piss or the PI as in something to do with a circle??

  • rate this

    Comment number 400.

    9. mcswaggerty

    "Something that cannot be seen, or heard. Something that has no apparent mass, and can pass unimpinged through anything and everything undetected.

    Sounds almost impossible to prove its existence."

    A sky wizard?!

  • rate this

    Comment number 399.


    @389 does being a 'multiverse person' imply that you are a 'quantum being'?

    Seriously - I believe that a cosmological model of a single big bank is highly improbable and that it is far more probably that the cosmos is the result of many big bangs in space, time & the other 7 dimensions!

    not seriously - stop taking the pi.pi!

    Physics & Maths do go in extraordinary directions!

  • rate this

    Comment number 398.

    @397 Dear Serk, I laughed out loud with sheer giddy panic..

  • rate this

    Comment number 397.

    Dark Matter is made of pure evil, if you manage to find it you will be instantly sucked into a vortex of eternal chaos and misery. It is no great mystery and many people in the past have found it and ended up imploding and being shot across the universe in a zillion invisible pieces. Ah no sorry that was a film I watched once. Carry on folks nothing to see here!

  • rate this

    Comment number 396.


    Intuitively in a multiverse model of reality with infinite number of big bangs the interactions of space and time are so horrendously complex that we haven't scratch the surface of what reality 'really' is!

    So I tend towards the idea that the 'invented-to-fit-our-half-knowledge' dark matter and dark energy just cover up the faults in our present model.

  • rate this

    Comment number 395.

    @389 John - does being a 'multiverse person' imply that you are a 'quantum being'?

  • rate this

    Comment number 394.

    392.Drunken Hobo

    Is is not string theory that bothers my maths it is the consequence of translating the string through a plane - mbrane theory. It really is pretty amazing that from 11 dimensional space we can conjure up a set of models that links most all observed states of matter and forces together and predicts that others should exist - like the higgs etc.

    Dark matter seems insubstantial!

  • rate this

    Comment number 393.


    Dark matter & entropy.

    Entropy - the second law of thermodynamics

    Dark matter - a postulated substance due to the failure of forces like gravity in explaining cosmological observations.

    So 'no' is the short answer. Long answer - tricky!

    See also the entropy hole that is postulated shortly after the big bang (I am a multiverse person so it becomes unimaginably more complex.)

  • rate this

    Comment number 392.

    390 John - I wouldn't fret about string theory. It's a load of dingo's kidneys.
    It's pretty much untestable so I'm not sure what use it is as a physical theory - it probably shouldn't even be called a theory until there is some evidence.

  • rate this

    Comment number 391.

    @390 As I said I'm still wondering if there is 'meant' to be some kind of correlation between 'dark matter' and 'entropy'. Also I am alarmed that Peer Gynt seemed to think there was some truth, apparently, in the Folklore from which he developed his tale. Could the 'to thine own self be true' thing have anything to do with the necessary development of some OCD faculty??

  • rate this

    Comment number 390.

    Reality is far far more peculiar that any of us imagine. And I include Physics & Mathematicians. I was going to say 'stranger', but that might confuse with the up, down, charm, strange, top & bottom quarks so I didn't & don't even go near strange matter!

    I am happy enough with 10 dimension string theory and can just about cope with 11 dimension mbrane - but all the interactions are beyond me!

  • rate this

    Comment number 389.

    @387 John - to try to use more scientific type language - is there any correlation between 'dark matter' and 'entropy'????

  • rate this

    Comment number 388.

    Its pretty clear that most people, men in particular, have a serious problem with the idea that they may not be right all of the time. Hence these Comments are filled with denial, and invoke a deity [supernatural, unprovable]. It's simply because those wearing the blinkers of certainty struggle to accept the fundamental process in science which says that every theory can be challenged

  • rate this

    Comment number 387.


    Red shift is the observation that the know atomic transmission lines of a spectrum of known elements are observed to be shifted towards the red end of the spectrum and from this we infer both time, speed and distance of distant objects. (look up the wiki if you are interested.)

    Nothing to do with red underwear either!

  • rate this

    Comment number 386.

    5 Minutes ago
    We're not going to learn a lot from this waste of money. Physics is a busted flush which has time and time again failed to deliver any real answers to the questions we all ask.

    Have you ever actually seen the end of your nose, blagshaw?

  • rate this

    Comment number 385.


    I favour a multiverse solution to the m-brane model but my maths/theoretical physics is not up to fully solving the 11 dimensional model and how our observations need to be explained in understanding all the consequences of accepting such a model on observations such as red shift - can anyone please explain?

    (I am not expecting any answer!)

    But my point is that we need more maths first!

  • rate this

    Comment number 384.

    @383 Forgive my ignorance - but has Red Shift got anything to do with that code at the end of that Alan Garner book - and something to do with a Church being burnt up by Lord Byron??

  • rate this

    Comment number 383.

    Dark energy and matter are required to explain why we perceive the universe the way that we do - importantly that the red shift idea is right. Perhaps it isn't and if it isn't then the dark bits and pieces may not exist as such.

    We 'may' have the Higgs Boson and the Standard Model, but I can't help thinking that there are still huge gaps in our understanding of the mbrane 11 dimensional model.


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