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 102.


    Yup, that would explain the confusion - must confess to not being as au fait with Strong Theory as perhaps I ought to be!

  • rate this

    Comment number 101.


    I agree with you, reasoned debate, and even thinking outside the box is not encoureged. Rather passive unquestiong acceptence of everything presented as fact. It turns people iinto something akin to sheep, which makes them much easy to manage, need I mention politics and advertising. How is that for a bit of dark energy?

  • rate this

    Comment number 100.

    I would never walk into a place of worship and disrupt a service by shouting about darwin. I therefore take exception to someone who believes that the bible has all the answers disrupting a discussion on science.
    I am also confused as to why someone who places so much faith in the bible would even read an article in the science section of a news website in the first place. Troll? Doubt?

  • Comment number 99.

    This comment was removed because the moderators found it broke the house rules. Explain.

  • rate this

    Comment number 98.

    Experiments are good. At least we are not spending money on carpet bombing other countries. I strikes me though, that when I hear a presentation on quantum physics, Physicists do not seems to have a clue what they are talking about. They all seem so pompous and self important, using flashy words that ordinary people do not understand, then they say: "the standard model might be wrong".

  • rate this

    Comment number 97.

    Oh yes...the holy trinity of dark matter, dark energy and dark flow...is to scientistics what the Father, the Son and the Holy Spirit is to theologists!

  • rate this

    Comment number 96.

    I can't help find amusing that many respected phyisicists have spent decades working on this problem,

    yet some of the ever-optimistic users of this comments page seem to think that they can put put forwards credible & coherent arguments against the current theory in the limited space allowed on this page.

    Good Luck.

  • rate this

    Comment number 95.

    If only more of our "brightest and best graduates" were working in areas like this - and paid a salary which reflects the level of skill involved - instead of wasting their abilities in the finance sector.

  • rate this

    Comment number 94.

    Only here could you get a synopsis of important research by a respected institute voted negative.

    #83, would love to here some more details! Go for it!

  • rate this

    Comment number 93.

    "Bovver Boy
    Darwinism, and the theory that we all come from monkeys. Whoops a theory, but presented as fact"

    You confuse terminology. A theory isn't an uncertain fact. Facts are derived from evidence; theories explain the facts. Evolution is an observed fact; Darwin's Theory explains how evolution works and shows that monkeys and man have a common ancestor (not we're descended from monkeys).

  • rate this

    Comment number 92.

    77.Daniel Bunbury

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

    Huh? it never was. Debate itself was always more towards legal thinking; taught for the defence or refutation of an idea regardless of its validity or your own belief either way.
    It would be good of schools encourage rational thinking wouldn't it?

  • rate this

    Comment number 91.

    Dark Matter and Dark Energy are two of the things that we know we dont understand about the universe yet and for this reason the search to understand them by experiment is as much at the peak of human scientific understanding as the hunt for the Higgs Boson. This is very exciting stuff if you appreciate this! Great to see such an informative article on the subject on the BBC.

  • rate this

    Comment number 90.

    I can't help but think that all the fools (today and historically) who say that physics and the other sciences are a waste of time have done nothing but hold back the technological advancement of the human race over the centuries.

    Shame on them for their ignorance.

    Think where we could be today if more people got behind science?

  • rate this

    Comment number 89.

    Comments upon abstractions of concepts drawn from models constructed from concepts of abstractions.

    A self-confessed utilitarian, I welcome the endeavours of those tormented souls who delve into the pointlessness of theoretical physics on the basis that very occasionally, there is crossover into the applied sciences and humanity benefits in real terms.

    You never know.

  • rate this

    Comment number 88.

    Even if it doesn't work, they'be got a great facility for storing nuclear waste.

    Or is that a wee bit cynical

  • rate this

    Comment number 87.

    If you're after dark matter, try drinking a couple of bottles of Gonzo Porter.

  • rate this

    Comment number 86.

    67 blagshaw

    I dont agree physics is a busted flush, but you are attracting neg vibes from those brought upon Darwinism, and the theory that we all come from monkeys. Whoops a theory, but presented as fact, though many scientists now question it. But if people think that great great ....granddad was an ape , what else do you expect? It might explain the behaviour of politicians towards us.

  • rate this

    Comment number 85.

    Stubbed my toe on some dark matter, put the light on and it turned out to be a chair in my bedroom. Now I've discovered what it was am I a physicist? If so please send me a grant to further my research or, failing that, some candles or batteries for my torch.

    Who actually funds this 'research' anyway? Surely there must be more worthwhile things for money to be spent on. Poverty, food etc

  • rate this

    Comment number 84.

    You just don't get it do you?
    I don't have "an argument"
    What I have is an interest in ALL of the theories, the evidence supporting them and living long enough to see which is proven right.

    You on the other hand seem to have an interest in thumbing down any science you don't agree with and in having an argument.

    Years gone by you'd be a flat earther!

  • rate this

    Comment number 83.

    Current standard theory predicts that some 90% of the mass in the universe is dark matter, but so far no attempt has succeeded in finding it. There is a simple answer, they are looking in the wrong place.
    A much simpler and more logical solution than current theory is that dark matter is tachyonic FTL matter. - I am a scientist working on a heretical FTL version of relativity and core physics..


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