Scientific belief has held that dark matter, the strange substance that comprises more than one quarter of the universe’s energy, is invisible and does not interact with light. New research from the University of York challenges this assumption, and instead suggests that light may pick up a faint tint of measurable color when it passes through dark matter. Calculations performed by the researchers showed that, although dark matter particles have no direct coupling to photons, the light-dark matter cross-section is faint, but nonvanishing. The team further showed that the energy dependence of light scattering on dark matter should color the dark matter either red, in the case of weak dark matter, or blue, in the case of gravitational dark matter, when light is passing through it. The researchers’ findings could inform future experiments on dark matter and advance the development of telescopes that help direct and focus the study of this mysterious substance. The gravitational effects of dark matter help shape galaxies and hold them together. To date, the presence of dark matter can only be detected through this gravitational pull, and scientists have mostly dismissed the possibility of detecting it through light. But when the York team examined dark matter, it made a complex discovery that challenged long-held scientific thinking. A University of York study indicates that light could pick up a subtle tint of red or blue, depending on the type of dark matter through which light is scattering. Courtesy of the University of York. “It’s a fairly unusual question to ask in the scientific world, because most researchers would agree that dark matter is dark, but we have shown that even dark matter that is the darkest kind imaginable — it could still have a kind of color signature,” researcher Mikhail Bashkanov said. The team initially calculated how strongly photons could interact with dark matter particles. By comparing the predicted effects of interaction with astronomical data, they identified constraints on possible dark matter candidates. They considered two types of dark matter — that is, when a dark matter particle can interact weakly, and all its dynamics are defined by conventional standard model physics, and when a dark matter particle can interact only gravitationally. They performed calculations of light-dark matter scattering under these two scenarios. They observed that the light tended to scatter backward in cases where the dark matter particle interacted weakly, coloring dark matter in red. When the dark matter was purely gravitational, the light tended to scatter forward, coloring dark matter in blue. The energy dependences of the cross-sections also showed differences between the two scenarios. “It’s a fascinating idea, and what is even more exciting is that, under certain conditions, this ‘color’ might actually be detectable,” Bashkanov said. “With the right kind of next-generation telescopes, we could measure it. That means astronomy could tell us something completely new about the nature of dark matter, making the search for it much simpler.” Even if dark matter does not interact directly with light, it may still influence it indirectly through other particles. For example, the most prominent dark matter candidate, the weakly interacting massive particle (WIMP), does not interact strongly or electromagnetically, but only weakly and gravitationally. WIMPs could connect to light via a series of intermediate particles, such as the Higgs boson and the top quark. The researchers applied the “six handshake rule” — the theory that any two people on Earth are connected by just a few mutual acquaintances — to their study of dark matter. They theorized that a similar chain of connections may exist among particles. The next step for the researchers could be to confirm their findings. Understanding dark matter remains one of the biggest challenges in modern physics. This research could lead to a new way to detect an astrophysical material whose existence, so far, can only be detected through gravity. It could help steer future investigations into dark matter. The possibility that dark matter leaves a color fingerprint on light could influence the way that future telescopes are built. “Right now, scientists are spending billions building different experiments — some to find WIMPs, others to look for axions or dark photons,” Bashkanov said. “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.” The research was published in Physics Letters B (www.doi.org/10.1016/j.physletb.2025.139920).