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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  • rate this

    Comment number 82.

    Logic tells me that the best place to find dark matter is somewhere dark. Just walk tentatively to minimise the pain caused by bumping into it.
    Similarly, I would look for wimps and machos on a beach. The wimps will be the ones with sand in their faces.

  • rate this

    Comment number 81.

    I don't understand why my previous comment has been the lowest rating on this topic.

    Oh well, let me explain.

    Dark matter is a likely but yet to be demonstrated solution to help explain the universe.
    "god" is an unlikely and unnecessary idea that provides no explanation for anyhting iother than human credulity.

    I hope this helps.

  • rate this

    Comment number 80.


    Fewer experiments and much more Bible study might see some real progress but where's the funding?
    Have you evidence that (your) God exists?
    If not, then can you suggest an experiment that will support your theory that God exists?
    If you cannot say yes to either you're unlikely to attract any funds.

  • rate this

    Comment number 79.

    And the real question is WHY God is doing stuff. Answers only Theology can give."

    And why, if his primary goal was to produce a species that worshiped him and his creations, he did so in an extremely inefficient, ultimately undeterministic and LOOOOONNNGGG process. I'm not convinced a deep study of the Bible will reveal much at all.

  • rate this

    Comment number 78.

    66. Fuzzy

    If physics wasn't held back by hundreds of years of religious wars and turmoil, I am sure we would be much further ahead.

    Physics = Answers
    God = Blind faith in something because you don't know the answer.

    I'm happy not knowing all the answers (just yet) and keeping an open mind to what might be out there.

  • rate this

    Comment number 77.

    Virtually every science story seems to be hijacked by fundamentalist and atheist nutters, each trying to beat the others' heads in. And the non-science stories get hijacked by the lefty-righty-nutters.

    Reasoned and considerate debate is apparently not taught in schools anymore.

  • rate this

    Comment number 76.

    68 Do you mean Prof. Stephen Hawking at the University of Cambridge, or Justin Hawkins from 'The Darkness'?!

  • rate this

    Comment number 75.

    And as I said, even models which give an optimistically large contribution for baryonic matter (MACHOS) have been shown to underestimate the amount of mass/energy in the universe.
    At no point did I say that the missing matter must be WIMPs.
    Mentioning neutrino's and parallel universes helps your argument how?

  • rate this

    Comment number 74.

    Last summer the ESO published research where they (believing in dark matter) mapped the motions of 100's of stars in the milky way to work out exactly how much and where the dark matter is.

    The results?

    There is none.

    These observations will be followed up by ESA's Gaia mission. Which I await with as much interest as the experiments in this article.

  • rate this

    Comment number 73.

    67. "Fewer experiments and much more Bible study might see some real progress but where's the funding?"

    Can't tell if trolling or just...

  • rate this

    Comment number 72.

    re: blagshaw "Physics is a busted flush "
    12.Jack Napier - Yeah that 'electricity' rubbish - 15.Trout Mask Replica - Sure, your computer functions because of incantations from a holy book

    These are works of the devil.
    Gods plan is clear:
    Live short, painful, behaviourally oppressed lives and proceed to heaven asap. Any deviation from that is pure evil.
    Switch off and suffer now !

  • rate this

    Comment number 71.

    Just because we can't touch it or see it does not mean it doesn't exist.
    So a bit like 'Fergie' time then, when ManU are losing or drawing?

    If I had a penny for every one of these particles that pass through me, I'd be a millionaire!

  • rate this

    Comment number 70.

    46. alexicon
    Qute right too!. Now where can i contribute to the "erase all religions" fund. I will gladly pay to be rid of christian and all other theology so we can then provide some answers without interruptions from the
    Call me an old fashioned romantic but a choice between adam and eve copulating and a newt like organism pulling itself from the ocean in search of guinness is a no brainer

  • rate this

    Comment number 69.

    I don't miss your point but you miss mine & perhaps that of Prof Ragazzi: "We may find that we have the wrong hypothesis..[dark matter] may be something completely different." Ergo you cannot be sure or fairly sure as to what 'Dark Matter' is, or is not; & how, if it exists, it's use may be abused by others in the future! Unless, & unlike Rutherford, you own a crystal ball!

  • rate this

    Comment number 68.


    Hawkings who wrote that book about time, is also credited with saying "Philosophy is dead, physics will be able to explain everything" But what is the everything that physics will be able to explain? It wonder if that was a defence mechanism because Hawkings doesn't know the answer himself, but it sounds good. But then if Hawkings says it, it must be true. Hmmm,

  • rate this

    Comment number 67.

    I don't understand why my previous comment has been the lowest rating on this topic.

    Why can't you science geeks understand that all you can show is HOW God is doing stuff. And the real question is WHY God is doing stuff. Answers only Theology can give.

    Fewer experiments and much more Bible study might see some real progress but where's the funding?

  • rate this

    Comment number 66.


    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.

    More money needs to be spent on Christian Theology which can actually provide some answers.

  • rate this

    Comment number 65.

    I believe it will turn out that today's dark matter is yesterday's ether, and the flat earth before that - something created in the minds of men to explain something they can't yet grasp. Waiting for the next Einstein...

  • rate this

    Comment number 64.

    "We know there is dark matter - but we have to understand what this dark matter is made of."
    No, they don't KNOW that there is dark matter - it's just that current theories - which will probably change loads of times! - don't work without it. HAVE TO UNDERSTAND what it's made of? No, WANT to.

  • rate this

    Comment number 63.

    #61 there is last years paper in the European Physics journal about Neutrino's and parallel universes.

    there are the theories about MACHO's, such as Brown dwarfs I previously mentioned, listed on the NASA website as a possible explanation, but hey I guess they know less than you?

    Finally the article "dark matter may be something completely different"

    You shouldn't need spoonfeeding!


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