THE identity of the universe鈥檚 dark matter may finally have been discovered. In what seems to be the most convincing claim for dark matter so far, researchers in England and France say gamma rays coming from the centre of our galaxy show hallmarks of these ghostly particles.
The research has only just been made public, so the team is still waiting for a response from other dark matter experts. But though the researchers are cautious, there is no hiding their excitement. 鈥淚鈥檝e dropped everything else to work on this,鈥� says Dan Hooper of the University of Oxford. 鈥淲e鈥檙e really excited,鈥� adds his colleague C茅line Boehm, also of Oxford. 鈥淚鈥檓 cautious but it鈥檚 surprising everything fits so well.鈥�
The identity of the universe鈥檚 dark matter, which outweighs the visible stuff by at least a factor of seven, is the outstanding mystery of modern astronomy. 快猫短视频s think it must exist because its gravity affects the way galaxies hold together. But the particles don鈥檛 emit any electromagnetic radiation so they have never been detected directly. No one knows what the particles are like, or exactly how they are distributed.
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However, because dark matter 鈥渇eels鈥� gravity like ordinary visible matter, it is a fair bet that it clumps in the centre of our galaxy. So the team turned their attention to a distinctive pattern of gamma rays coming from the centre of the Milky Way (see Graphic). The sharp signal, which has an energy of 511 kiloelectronvolts (keV), is believed to be due to the annihilation of electrons and positrons 鈥� the antimatter equivalent of electrons.
But where did the electrons and positrons come from? People have speculated that the source is anything from the blast waves of a 鈥渉ypernova鈥� 鈥� a super-powerful supernova 鈥� to a neutron star or black hole. 鈥淏ut none of the explanations have seemed satisfactory,鈥� says Hooper.
The researchers wondered whether the electrons and positrons might in fact come from the annihilation of dark matter particles and their antiparticles at the centre of the galaxy. But to produce a sharp line at 511 keV 鈥� which is the 鈥渞est energy鈥� of an electron 鈥� the electrons and positrons must be slowed to a virtual standstill before they annihilate each other, ruling out dark matter at the large masses most researchers expect. 鈥淗eavy dark matter particles would produce high-energy electrons,鈥� says Hooper. 鈥淪ince it鈥檚 difficult to imagine how they could be slowed to a standstill, we were forced to consider a surprisingly light dark matter particle.鈥�
By 鈥渓ight鈥�, the researchers mean 1 to 100 megaelectronvolts, which is between 1000 and 10 times lighter than a proton. Such a light particle is surprising because particle accelerators routinely create particles of this mass, so the particle should have revealed itself. 鈥淭o have escaped detection, it must be very weakly interacting,鈥� says Hooper. 鈥淎 particle in [this] range could have been missed,鈥� agrees Nigel Smith, head of the UK Dark Matter Collaboration Experiment.
To test their idea, the researchers looked at observations from Integral, the European Space Agency鈥檚 gamma-ray telescope. Launched in October 2002, Integral has made the most precise measurements yet of the 511 keV line and mapped changes in its brightness across the central bulge of our galaxy. Crucially, the team found that the brightness map fits exactly with the distribution that would be expected of their light dark matter particles.
If dark matter really is made up of such light particles, every cubic centimetre of space in the vicinity of the Earth must contain a few tens of them. So you should be able to detect them in lab-based experiments. 鈥淭he claim would become much more interesting if a particle or nuclear physics experiment finds a new particle with the properties the team suggest,鈥� says Ben Allanach of CERN, the European centre for particle physics.
Teams hunting for dark matter on Earth usually focus on much more massive particles 鈥� bigger than 10 gigaelectronvolts 鈥� by trying to detect the recoil of an atomic nucleus hit by a dark matter particle. Silk and his colleagues are now looking into whether any existing experiment might show evidence of the new particle or whether any could be easily modified to detect it. The researchers plan to submit their paper, available at , to Physical Review Letters.