Scientists find 'evidence of dark matter'

They seem to bear the signature of collisions between atoms of dark matter, the mysterious “stuff” that cannot be seen or detected directly but which binds the cosmos together.

However, the possibility that the particles have some other origin cannot yet be discounted. Over the next few months, further analysis will show whether the “smoking gun” of dark matter really has been discovered.

The seven ton AMS is a super-sophisticated particle collector attached to the outside of the International Space Station.

Since its installation in May last year it has been gathering data from millions of light years beyond our galaxy, the Milky Way.

Until now, scientists have only been able to theorise the existence of dark matter, which makes up around 26% of the universe.

Its ghostly presence around galaxies exerts a gravitational effect that can be measured, but what it is made of remains to be proven.

A leading theory suggests that dark matter is composed of exotic particles known as Wimps (weakly interacting massive particles).

If Wimps exist, they would annihilate each other when they collide to release electrons and their antimatter equivalent, positrons.

It is the positrons left behind by dark matter collisions that AMS is looking for. By analysing the ratio of positrons to electrons and measuring the energy of the particles, scientists hope to find the first solid clues to the nature of dark matter.

A rise in the positron energy followed by a dramatic fall is what would be expected from Wimp collisions.

Another telling sign is the direction the positrons are coming from. If they are generated by dark matter, they should be spread evenly through space. But if they are created by a normal process, such as an exploding star, they would originate from a single direction.

The findings announced by the American space agency Nasa today reveal a positron spike “consistent” with Wimp annihilation.

No preferred incoming direction for the particles was seen, which is also indicative of dark matter.

But the scientists say the evidence is not yet “sufficiently conclusive” to rule out other explanations.

AMS principal investigator Professor Samuel Ting said: “Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin.”

The results are based on some 25 billion recorded events, including 400,000 positrons – the largest collection of antimatter particles captured in space.

Another potential source of the positrons could be pulsars – rotating super-dense neutron stars that shoot out radiation beams like cosmic lighthouses - distributed around the flattened disc of the Milky Way.

“When you take a new precision instrument into a new regime, you tend to see many new results, and we hope this will be the first of many,” said Prof Ting. “AMS is the first experiment to measure to 1% accuracy in space. It is this level of precision that will allow us to tell whether our current positron observation has a dark matter or a pulsar origin.”

Other searches for dark matter are taking place at the Large Hadron Collider particle accelerator near Geneva and in laboratories buried deep underground.

British expert Dr Chamkaur Ghag, from University College London, said: “It’s not a smoking gun, but what it is is a tantalising glimmer of the existence of dark matter.

“What they’re seeing is more or less what you’d expect from dark matter if it’s out there. The difficulty is assigning it to dark matter and not to other known astrophysical sources.

“Overall, we’re certainly seeing something; maybe lots of dark matter producing this positron excess, but we’re certainly not able to see it clearly enough yet. It may well be dark matter but it will take a bit of work to be confident about that.”

Findings from AMS could help other scientists trying to detect dark matter by showing them what to look for, he said.

Dark matter particles have no place in the Standard Model, the grand theory that explains how all the basic building blocks of matter interact.

“It’s a window telling us that there’s something beyond the Standard Model of particle physics,” said Dr Ghag. “Dark matter shows us that the story doesn’t end here – there’s more to the universe.”

Ordinary matter, the kind that makes up stars, planets and ourselves, only accounts for just 4% of the universe, scientists now believe.

The remaining 70% of the cosmos is thought to consist of dark energy, an even bigger enigma than dark matter, which appears to be driving galaxies apart at an accelerating rate.