Is it a false alarm or a discovery that solves one of the greatest mysteries in cosmology? This is the question weighing on astronomers as they examine a study published recently in the Journal of Cosmology and Astroparticle Physics, which claims to have finally detected the elusive ‘dark matter’.
Dark matter is believed to have been around for most of the 14-billion-year history of the universe. Astronomers began searching for it in the early 1930s after the Swiss astronomer Fritz Zwicky observed that galaxies in the Coma Cluster were moving too quickly for the amount of ordinary matter it contained. He realised the speed of their rotation was so great that they should have flown apart as they didn’t have enough matter to generate the gravity needed to hold them together. He deduced that some hidden mass could be providing the ‘extra gravity’ required for the galaxies to stay intact. He named this dark matter.
Invisible WIMPs
According to the Standard Model of particle physics, ordinary (baryonic) matter that makes up the world around us consists of elementary particles such as baryons (protons and neutrons) and electrons, along with the massless photons of electromagnetic radiation such as light. Baryons themselves are made up of even smaller particles called quarks and gluons. But all these fundamental particles form just 5% of all mass in the known universe. Dark matter accounts for 27%, while a mysterious force called ‘dark energy’ makes up the rest.
Physicists don’t know what dark matter is made up of, but one hypothesis they have is a hitherto unknown type of subatomic particle called WIMPs. The name is short for ‘weakly interacting massive particles’. According to physicists, WIMPs barely interact with normal matter and not at all with any form of electromagnetic radiation. Since dark matter doesn’t emit, absorb or reflect light, astronomers can only study its gravitational effect on visible matter, such as stars and galaxies.
The trick to finding it then is to spot its tell-tale signature: high-energy particles, such as gamma-ray photons that are released when two WIMPs collide and annihilate each other.
New claim
Tomonori Totani of the University of Tokyo has now claimed to have identified just such a gamma-ray signal in his study, using data from the Fermi Gamma-ray Space Telescope.
“We detected gamma rays with a photon energy of 20 giga-electron-volts (or 20 billion electron-volts, an extremely large amount of energy) extending in a halo-like structure towards the centre of the Milky Way galaxy” Prof. Totani said. “The gamma-ray emission component closely matches the shape expected from the dark matter halo.”
He added that the measured gamma ray energy spectrum “closely matches model predictions for the annihilation of hypothetical WIMPs with masses roughly 500-times that of a proton.”
Astronomers always knew dark matter must be virtually hiding in plain sight and that they’d find it sooner or later. Has Prof. Totani found it, however? Not quite, say experts, as the research data have to survive rigorous scrutiny and critical evaluation by more independent researchers.
‘Excess’ radiation
“When we see a signal that looks like it could be dark matter, we can check other regions that are rich in dark matter to look for a comparable signal there,” Tracy Slatyer, professor of physics and Director, MIT Centre for Theoretical Physics, told this correspondent via email.
“Studying the detailed properties of the signal would tell if it’s consistent with what we expect from dark matter or if it has traits more consistent with an alternative source. So far, we have had many signals that seemed at first glance like to be dark matter, but subsequent in-depth analysis revealed they were from a different source. Moreover, the overall size of the signal is not what you would expect from classic WIMP models (it is too large), and when we do the test of looking at other dark-matter-rich regions, we do not see a corresponding signal.”
According to Rishi Khatri of the Department of Theoretical Physics at the Tata Institute of Fundamental Research, Mumbai, the findings suggest the detection of an excess of radiation compared to what cosmologists expect from the model of the Milky Way galaxy.
An illustration of the two gigantic X-ray/gamma-ray bubbles (blue-violet) known to be associated with the Milky Way (centre).
| Photo Credit:
NASA
“It is possible that this excess is just pointing to something missing in the model of our galaxy rather than dark matter.” Prof. Khatri said in an email interview. “Based on what the study claims, we can predict what kind of signal we can expect from other nearby galaxies and then try to observe these signals. There have been similar claims many times in the past about detection of dark matter but which turned out to be false.”
Radiation from other sources
Discoveries in particle physics have to typically reach a confidence level called ‘5 sigma’ before they are considered valid. Where does the new finding stand on this scale?
“The excess reported in the paper seems to be quite more than 5-sigma, without taking into account the uncertainty in modelling,” Prof. Khatri said. “The author has not given a number with the modelling uncertainty included. This should give you an idea about how uncertain we are about the uncertainty (i.e. error on the error bars) which is very important in such studies.”
An immediate task for astronomers would be to rule out the possibility of the radiation coming from some other sources of high-energy radiation like supernovae, the explosive deaths of massive stars; neutron stars, the ultra-dense collapsed cores of massive stars after supernova explosions; or black holes.
The gravity of dark matter, like normal matter, should cause the light passing nearby to bend, in a phenomenon called gravitational lensing. A spectacular example of this is the Bullet Cluster, where the collision of two clusters of galaxies resulted in the separation of dark matter from normal matter, and astronomers could discern the halos of dark matter around the galaxies by how they bent the path of light.
The LCDM model
If the findings stand up to scrutiny and it turns out that a dark matter particle has actually been found, the widely accepted Lambda-Cold Dark Matter model of the universe wouldn’t have to be modified. This is because, as Prof. Khatri said, “A new particle that would form dark matter is actually included in the LCDM model.”
Instead, “what we do not know is the exact nature of dark matter.”
And “if the dark matter is a 500 GeV WIMP as is claimed, it would be expected to have only very tiny interactions with other dark matter particles or with ordinary matter,” Prof. Slatyer said. “So it would be safe to treat it as having only gravitational interactions, and many of the predictions of LCDM could remain almost unchanged.”
One thing that can be said with certainty is that this grand cosmic narrative is at an exciting stage as astronomers untangle the fabric of the universe to try to understand its evolution and nature.
Prakash Chandra is a science writer.
Published – December 18, 2025 05:30 am IST
