3 July 2012

What is dark matter?

By
If there's anything harder to find than the Higgs boson, it's dark matter. It could make up more than 80% of the universe but hasn't been seen yet - physicists are looking for it in space, deep underground in old mines, and in huge particle accelerators, such as the Large Hadron Collider at CERN.
what is dark matter?

This image, taken by the Hubble Space Telescope, shows the galaxy cluster CL0024+17. In the centre, there are several unusual and repeated galaxy shapes - these are distant galaxies bent by the gravitational force of a ring of dark matter. The ring, five million light years across, has been super-imposed in blue. Credit: NASA

What is dark matter?

Dark matter is one of the most perplexing puzzles in modern physics, and was first postulated in the 1930s by Dutch astronomer Jan Oort and Swiss astrophysicist Fritz Zwicky.

Calculations show that it is five times more abundant in the universe than standard matter, but since it doesn’t emit, absorb, or reflect light, it is very tricky to spot. In fact, its existence can only be inferred from the gravitational force it exerts on its surroundings.

What evidence is there for its existence?

Stars and galaxies don’t move how we think they should. Scientists observing the motion of galaxies noticed in the 1930s that they rotate much faster than their observable masses should allow; at these speeds, they ought to be flung apart, unable to produce enough gravity to hold together under their centrifugal force. Galaxies also ought to rotate faster at the centre, like water in a plughole, and yet they don’t – in the 1970s, astronomers found that they move more solid discs or blobs.

These intriguing observations could be explained by the presence of large quantities of additional invisible mass.

The extent to which light is bent and deflected around clusters of galaxies – so-called ‘gravitational lensing’ – provides further compelling evidence for the presence of dark matter in addition to the visible matter contained in clusters.

Why is finding it important?

Dark matter appears to make up more than 80% of the matter in our universe. On the other hand, some scientists adamantly believe that it is not dark matter causing the gravitational effects that have been attributed to it. They wonder instead whether we misunderstand how gravity works at long range, and suggest that our established theories of gravity may need an overhaul.

Because dark matter seems to interact so weakly with other particles, cosmologists think it could have been around since the very earliest moments of the universe and had a critical role in defining its structure.
How can we detect it?

How do scientists look for proof of dark matter?

Direct detection experiments essentially comprise large chunks of metal held in underground mines. The theory goes that, even if dark matter particles only interact very weakly with normal matter, if they really are so ubiquitous then given enough time and patience we should eventually witness one bump into a particle of normal matter. These experiments are sitting, waiting, and watching for these rare and subtle signs.

Indirect searches look out to space, hunting for evidence of dark matter decaying, annihilating, or being eaten up by the black holes at the centres of quasars, for example.

Colliders, such as the Large Hadron Collider in Geneva, Switzerland, operate at such high energies that they could conceivably produce dark matter particles. Although they would be technically invisible to the huge detectors, they could be identified by the conspicuous amounts of ‘missing energy’ that they carry out of the detectors.

What do the latest searches show?

To date, no convincing empirical evidence has been found, although a German theorist recently claimed to spot evidence of dark matter annihilation in publically available data from the Fermi gamma-ray telescope.

Searches continue, with the LHC having the best chance of creating and detecting low-mass dark matter candidates (under 10-100 GeV), and direct detection experiments such as XENON100 at Gran Sasso, Italy, and the Ice Cube Neutrino Observatory at the South Pole more sensitive to high-mass candidates.

If dark matter hardly interacts with anything, will we ever truly ‘see’ it?
Not necessarily. If dark matter turns out only to interact gravitationally, then we will only ever be able to infer its existence by looking at its galactic scale effects from far away…

Ceri Perkins is a freelance journalist based in Geneva, Switzerland.
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