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 302.

    A terrible and remarkable thing happened that went largely unnoticed in the news recently.Physicists' predictions they could ignite a fusion reaction with 192 of the world's most powerful lasers focused on a 2mm spot didn't work.Experiment cost 3.7 billion dollars.It's the same math used to assure reliability of America's nuclear deterrence.Maybe it's time to start actually testing them again.

  • rate this

    Comment number 301.

    It is absolutely amazing, mind boggling what scientists 'think' and then 'achieve'. I'm not scientifically inclined unfortunately but think I would like answers which lie closer to home or our day to day existence.

  • rate this

    Comment number 300.

    I remember reading about someone asking if Fermilab in the USA contributed to its defense. Only that it helps make it worth defending was the reply. Its the same with all this kind of science, knowing the makeup of 96% of the Universe wont benefit us directly, but science for its own sake leads to all sorts of advances we all benefit from.

  • rate this

    Comment number 299.

    I saw a video yesterday regarding a massive field of quasars 4 billion light years across supposedly) calling into question redshift correlation with distance vindicating Halton Arp theories and consequently calling into question the big bang from which the theory of dark matter originates. It think the Primer Fields videos on Youtube propose a much more elegant idea

  • rate this

    Comment number 298.

    293 Vampire - The prospect of general relativity is what caused me to abandon physics at university. Special relativity was hard enough, but general relativity looked unfathomable. It makes quantum mechanics look friendly!

  • rate this

    Comment number 297.

    293 It is often very difficult to use or create mathematical models to describe in precise detail physical concepts. Sometimes classical math just doesn't quite fit. But ultimately these equations have to be related to our notions of time, space, matter, energy even if what our senses tell us is badly distorted and must be rectified.Without that explanation equations in a vacuum tell us nothing.

  • rate this

    Comment number 296.

    Feynman, Sagan, Einstein would be proud.

  • rate this

    Comment number 295.


    ". . .Everything on Star Trek is possible. . ."

    Except the fact that Captain Kirk copped off with a different alien every episode, which is pushing reality a bit. . .

    I can't help thinking, too that if the stuff they were looking for was given a less sexy name, like "wobba-doo" then the yarboo-sucks God/science slanging match might not have kicked off so tragically!

  • rate this

    Comment number 294.

    No mention of the scientists attempting this deep under the sea just off the Cleveland/North Yorks coast in the bowels of Boulby Potash Mine (BBC reported Jan 2010).

    My brain hurts thinking about it!

  • rate this

    Comment number 293.

    General relativity is in a completely different animal to special relativity. Special relativity is fairly simple and covered in most 1st year undergrad courses. General relativity is far more complex and in many cases isn’t covered in undergrad at all. Einstein had to practically invent some of the tensor maths to get it to work. No wonder many people don’t get it…it’s really difficult!

  • rate this

    Comment number 292.

    288"I have recently read a book on Einstein's General Theory of Relativity. Did you know that only about 3,000 people in the whole world truly understand the theory?"

    Hard to accept.My understanding of Special Theory is better.Proof of General Theory was 1919 eclipse experiment.It proves photons are attracted by gravity but not why.Curviture of space expl only works if photons have no mass.

  • rate this

    Comment number 291.

    Also scientists are so closed minded. If it's not measurable it does not exist. Better look at the Scole report and what's happening in reality than chasing own tail - ancient Welsh saying.

  • rate this

    Comment number 290.

    I'm a bit sceptical of this experiment. How will they know that light from an excited argon atom is caused by dark matter and not by something else? In my view "dark" matter/energy is a bit of a mis-noma. It should really be called "matter/energy we don't understand". So how do we know we have the right bit of not-understood matter, the same as causes the unaccounted for mass in space?

  • rate this

    Comment number 289.

    286.sieuarlu - "......I understand it, I just don't believe it because it makes no sense........."

    And therein lies one of your problems. Science isn't about belief, that's for religions & homeopathy et al.

    Inherant in something being a theory means it hasn't been definitively porven yet, hence why no genuine scientist says things are facts when they are yet to be proven either way.....

  • rate this

    Comment number 288.

    Fascinating. I eagerly await the results of this experiment and look forward to reading one of the many 'Popular Science' books that are sure to be written on this subject.

    I have recently read a book on Einstein's General Theory of Relativity. Did you know that only about 3,000 people in the whole world truly understand the theory?

    Alas, I am still not one of them.

  • rate this

    Comment number 287.

    In the BBC news report - why use a picture of a planetary nebula while describing gravitational lensing? The pretty space pictures used had nothing to do with the narrative. I appreciate the subject matter is fairly heavy but your news article is supposed to be educating people; the imagery used is misleading and incorrect, there are plenty awesome images of gravitational lensing. TY : )

  • rate this

    Comment number 286.

    264"Actually they do understand those things, though you may not understand the explanations."

    A result of my traning I have the capacity to understand their theories.They went from string theory to membrane theory to???From one preposterous explanation to another.I understand it, I just don't believe it because it makes no sense.Who are they, priests of some new religion who monopolize truth?

  • rate this

    Comment number 285.


    Now how about the dark side of the fiscal problems of the UK?

  • rate this

    Comment number 284.

    For 60 years or more physicists have been smashing atomic particles together with increasing energy watching the ashes of the collision that may exist for only billionths of a second to understand matter and energy.IMO it's like crashing 2 planes in mid air and watching what falls from the sky to understand principles of aerodynamics and what makes planes able to fly.Results are as expected, poor.

  • rate this

    Comment number 283.

    People ask 'How will this benefit us?'

    The story goes that Maxwell was once asked by a government VIP "What use is it?" about electromagnetism.

    He said either "What use is a new born baby> or " I dont know, but one day you will tax it"


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