Nearly a century ago, scientists proposed that a mysterious invisible substance they named dark matter clumped around galaxies and formed a cosmic web across the universe.
What dark matter is made from, and whether it is even real, are still open questions, but according to a study, the first direct evidence of the substance may finally have been glimpsed.
More work is needed to rule out less exotic explanations, but if true, the discovery would go down as a turning point in the decades-long search for the elusive substance that is said to make up 27% of the cosmos.
“This could be a crucial breakthrough in unraveling the nature of dark matter,” said Prof Tomonori Totani, an astrophysicist at the University of Tokyo, who said gamma rays emanating from the centre of the Milky Way appeared to bear the signature of the substance.
Dark matter was first described in the 1930s, when the Swiss astronomer Fritz Zwicky noticed that distant galaxies appeared to be spinning faster than their mass allowed. The observations led to the notion of dark matter, a material that neither emits nor absorbs light, but exerts an unseen gravitational pull on the galaxies it surrounds.
Scientists have searched for dark matter particles ever since, but so far ground-based detectors, space-based telescopes and vast machines such as the Large Hadron Collider near Geneva have drawn a blank.
One of the many theories of dark matter postulates that it is made from so-called weakly interacting massive particles, or wimps, which are heavier than the protons found inside atoms, but barely interact with normal matter. When two wimps collide, they can annihilate one another, releasing other particles and a burst of gamma rays.
To search for potential dark matter signals, Totani analysed data from Nasa’s Fermi Gamma-ray Space Telescope, which detects the most energetic photons in the electromagnetic spectrum. He spotted a pattern of gamma rays that appeared to match the shape of the dark matter halo that spreads out in a sphere from the heart of the galaxy.
The signal “closely matches the properties of gamma-ray radiation predicted to be emitted by dark matter,” Totani told the Guardian. Details are published in the Journal of Cosmology and Astroparticle Physics.
If Totani has seen dark matter at work, the observations suggest it is made from elementary particles 500 times more massive than the proton. But far more work is needed to rule out other astrophysical processes and background emissions that could explain the signals.
Totani said the “decisive factor” would be detecting gamma rays with the same spectrum from other regions of space, such as dwarf galaxies. According to Prof Justin Read, an astrophysicist at the University of Surrey, the lack of significant signals from such galaxies strongly argues against Totani having seen gamma rays emitted from dark matter particle annihilation.
Prof Kinwah Wu, a theoretical astrophysicist at UCL, also urged caution. “I appreciate the author’s hard work and dedication, but we need extraordinary evidence for an extraordinary claim,” he said. “This analysis has not reached this status yet. It is a piece of work which serves as an encouragement for the workers in the field to keep on pressing.”
