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 22.

    @6 blagshaw
    I'm sorry but I completely disagree, Physics has given us the world as we know it. Christian Theology would have said the world was flat and created in 7 days, Physics has shown strong evidence, not theory, that neither of these are true.

    When I read an article like this I find it sad people must complain about "wasted money" when everything we have stems from physics research

  • rate this

    Comment number 21.

    "...like forcing 2+2 to equal 100 and getting everyone to believe it because there’s this unknown "made up" factor that makes it true.

    ...time to rethink the standard model."

    On the surface, your point is ok, but it's more like a situation where if we have a bag, throw 2 beans in, then another 2, and when we open it, we find 100.

    Everything else fits, so the questions are "how" & "why"

  • rate this

    Comment number 20.

    #9 WIMP Weakly Interacting Massive Particle
    Dark matter has mass but it is electromagnetically invisible.

  • rate this

    Comment number 19.

    I find the dark matter search interesting.

    However there are alternative theories.

    More and more stuff is being found regading the numnbers of "stars" to cold to be seen, rogue stars, and even lone exoplanets. All of which are currently barely detecable.

    Some scientists believe things like this account for much of the "missing matter"

    And of course the standard model may be wrong!

  • rate this

    Comment number 18.

    Did anyone do an autopsy on Darth?

  • rate this

    Comment number 17.

    I'm not a physicist - just a fascinated observer. Forgive me if I'm showing ignorance here, but if that's what I'm doing, then please point out where I am wrong:
    (i) dark matter is virtually undetectable
    (ii) this "multiverse" supposedly comprises eleven different dimensions
    (iii) where and what are dimensions numbers 5 through eleven?
    Undetectable, aren't they?
    (iv) DM in different dimen?

  • rate this

    Comment number 16.

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

    That's right. We need to prove once and for all the Earth 5,000 years old, was created in a 6 days & is the centre of the Universe.

  • rate this

    Comment number 15.

    Physics is a busted flush which has time and time again failed to deliver any real answers to the questions we all ask"

    Sure, your computer functions because of incantations from a holy book rather than that "busted" quantum mechanics rubbish, right? Your sat nav works by supernatural intervention rather than the physics of general relativity? Your car works by.... Get the picture?

  • rate this

    Comment number 14.

    "Until now, the hunt for dark matter has proved elusive". Surely it is not the hunt but the matter that's been elusive? You guys should pay someone to check your copy.

  • rate this

    Comment number 13.

    As Mr Hawkins once noted ... Theories seem to have a built in defence mechanism , to stop us finding out what they are .

  • rate this

    Comment number 12.

    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.


    Yeah that 'electricity' rubbish - what a waste of time that turned aout to be.

    And don'tr get me started on utter wastes of time & money like CAT scans & X-ray machines.

  • rate this

    Comment number 11.

    The following may help fill in the background accurately to this without being too technical for those who are interested

  • rate this

    Comment number 10.

    Only a few years ago, black holes were just a theory. We have to find out what makes this universe tick. If we don't, the Human Race will stagnate & eventually die out as our Sun becomes too hot to support life in this Solar System.
    When we find out what Dark Matter is, it's a short step to find out about Dark Energy - and that could be really interesting.
    Re Post 6 - God gave us a brain to use.

  • rate this

    Comment number 9.

    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.

    Having said that, best of luck! If millions of these particles are passing through me every second it would be nice to know a bit more about them!

  • rate this

    Comment number 8.

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

    (admittedly all the answers are 'God' but that's not the point.)"

    Ahh the TROLLS are out again I see.

  • rate this

    Comment number 7.

    WIMPS,MACHOS-Dark matter whose existence has been established because of its gravitational effects on distant objects and it constitutes 23% of the universe.
    So 23% of all the stuff around us is undetectable,but heavy-Why does that seem like a contradiction? How can it not be detectable?

  • rate this

    Comment number 6.

    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.

    (admittedly all the answers are 'God' but that's not the point.)

  • rate this

    Comment number 5.

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

    If even the hunt is proving elusive, what hope can there possibly be of finding the bloody stuff?

  • rate this

    Comment number 4.

    Fascinating - is there a whole bunch of innovative research going on lately, or has the BBC just bucked up on its science coverage?

    Keep up the good work, I love reading about all this stuff!

  • rate this

    Comment number 3.

    @ Mike from Brum

    No scientist has ever claimed to "know it all". That's what science is; a constant quest for understanding and revision of existing knowledge.


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