By Will Dunham
WASHINGTON (Reuters) – Scientists may be getting closer to confirming the existence of dark matter, the invisible substance thought to make up more than a quarter of the cosmos, as they probe a diffuse gamma-ray glow near the center of our galaxy.
Everything in the visible universe is made of ordinary matter, from stars and planets to people, domes, and trails. Ordinary matter can be seen at wavelengths from infrared to visible light and gamma rays, but it only makes up about 5% of the universe. Dark matter, which does not absorb, reflect, or emit any light, appears to make up about 27% of the universe, while another mysterious component called dark energy makes up the remaining roughly 68%.
Scientists believe that dark matter exists because of its large-scale gravitational influence on the universe. Due to its nature, its existence was difficult to prove. But the gamma-ray excess observed and imaged by the Fermi Gamma-ray Space Telescope in the vast expanse of space near the heart of the Milky Way promises to provide the long-sought confirmation.
Scientists have offered two competing explanations for these gamma-ray emissions.
One is that they are caused by colliding dark matter particles concentrated in this region of the galaxy. Another is that they are caused by a class of neutron stars—the densely collapsed cores of massive stars after they die—called millisecond pulsars, which emit light across the electromagnetic spectrum as they spin hundreds of times per second.
A detailed new analysis, including advanced simulations, weighed the merits of these competing hypotheses, finding them equally likely. The research showed that gamma rays generated by the collision of dark matter particles would create the same gamma-ray signal observed by the Fermi satellite.
“Understanding the nature of the dark matter that pervades our galaxy and the entire universe is one of the biggest problems in physics,” said cosmologist Joseph Silk of the Johns Hopkins University in Maryland and the Institute of Astrophysics at the University of Paris/Sorbonne, one of the authors of the study published Thursday in the journal Physical Review Letters.
“Our main new result is that dark matter matches the gamma-ray data at least as well as the competing neutron star hypothesis. We have raised the possibility that dark matter was detected indirectly,” added Silk.
The researchers said the world’s most powerful ground-based gamma-ray telescope, the Cherenkov Telescope Array Observatory, currently under construction in Chile, could provide the answer by distinguishing between gamma rays from these two sources. It could start operating in 2026.
“Because dark matter does not emit or block light, we can only detect it through its gravitational effect on visible matter. Despite decades of searching, no experiment has yet directly detected dark matter particles,” said astrophysicist and study leader Moorits Mihkel Muru from the University of Tartu and the Leibniz Institute for Astrophysics in Potsdam.
The gamma-ray excess was observed in a region extending 7,000 light-years inside the galaxy. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). This region is approximately 26,000 light-years from Earth.
Gamma rays have the shortest wavelengths and the highest energy of any wave in the electromagnetic spectrum. Why might gamma rays be evidence of dark matter? Dark matter particles are suspected to completely annihilate themselves when they collide, and these collisions produce gamma rays as a byproduct.
The Milky Way is believed to have formed when a huge cloud of dark and ordinary matter collapsed under the influence of gravity.
“The ordinary matter cooled and fell into the central regions, pulling in some dark matter,” Silk said. “What’s unique about the simplest dark matter hypothesis is that dark matter particles are their own antiparticles and annihilate completely when they collide. Only protons and antiprotons do something like that to create energetic gamma rays, and antiprotons are extremely rare.”
But the glow could also be caused by many thousands of hitherto unobserved millisecond pulsars. The Fermi satellite confirmed that such objects are gamma-ray sources that could explain the glow in this region.
(Reporting by Will Dunham Editing by Rosalba O’Brien)