Not-so-dark matter? Mysterious substance might leave red and blue 'fingerprints' on light

Dark matter, one of the universe's best kept secrets, may have been quietly painting the cosmos in faint, detectable hues of red and blue all along, a new study suggests.

Dark matter makes up more than 80% of the matter in the universe, yet it doesn't emit, absorb, or reflect light, making it impossible to observe directly. Now, a new theoretical study by scientists at the University of York in the U.K. suggests light passing through dark-matter-rich regions of space could pick up a faint tint — slightly red or blue, depending on the kind of dark matter it encounters.

The effect would be extraordinarily subtle, far too weak for current telescopes to detect, but potentially measurable with the next generation of ultra-sensitive observatories, the researchers say.

"It's a fairly unusual question to ask in the scientific world, because most researchers would agree that dark matter is dark," study co-author Mikhail Bashkanov of the University of York said in a statement. "But we have shown that even dark matter that is the darkest kind imaginable — it could still have a kind of colour signature."

The team likens the concept to the "six handshakes rule," the 20th-century theory that any two people on Earth are connected by a chain of, at most, six acquaintances. In a similar way, the study suggests, even if dark matter doesn't interact directly with light, it might do so indirectly through intermediate particles that both sides "know," including the Higgs boson, the so-called "God particle" that represents the Higgs field, which is responsible for giving other particles their mass.

This indirect link could allow photons, the particles of light, to scatter ever so slightly off dark-matter particles, leaving behind a whisper of color or polarization "fingerprint" in the light, the study suggests.

"It's a fascinating idea, and what is even more exciting is that, under certain conditions, this 'colour' might actually be detectable," Bashkanov said in the statement. "With the right kind of next-generation telescopes, we could measure it."

In their study, published earlier this month in the journal Physics Letters B, Bashkanov and his team carried out what they say are the first detailed calculations of how strongly light could scatter off dark matter.

The findings suggest that if dark matter is made up of Weakly Interacting Massive Particles, or WIMPs, which interact through the weak nuclear force, then light passing through a WIMP-rich region would lose some of its high-energy blue photons first, leaving the transmitted light slightly red-tinted. In contrast, if dark matter interacts only through gravity, photons would scatter in the opposite way, giving the light a faint blue shift, the study notes.

In both situations, the interactions are minute but not zero, researchers say, meaning dark matter could leave behind a detectable "fingerprint" on light that travels through dense regions of it, such as the centers of galaxies or galaxy clusters.

Their calculations show that these effects could slightly distort the light spectrum of distant objects. A galaxy's glow, for instance, might appear microscopically redder or bluer depending on the dominant type of dark matter lying between it and Earth. In principle, such differences could help scientists distinguish between dark-matter models based on whether cosmic light skews red or blue as it travels through dark-matter-rich space.

"Right now, scientists are spending billions building different experiments — some to find WIMPs, others to look for axions or dark photons," Bashkanov said in the same statement. "Our results show we can narrow down where and how we should look in the sky, potentially saving time and helping to focus those efforts."

Detecting such tiny shifts would require ultra-precise telescopes and painstaking analysis of light that has traveled billions of light-years across the cosmos. Future observatories with exceptional spectral and polarization sensitivity, such as the European Extremely Large Telescope and NASA's Nancy Grace Roman Space Telescope, could one day test these predictions.

If confirmed, the findings would open an entirely new observational window on dark matter, bringing scientists a step closer to unraveling one of the greatest mysteries in cosmology.