‘Touchy-feely’ dark matter is having a moment

May 6, 2026

6 min read

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‘Touchy-feely’ dark matter is having a moment

Models giving dark matter more complex behavior could help solve multiple cosmic mysteries

By Paul M. Sutter edited by Lee Billings

gremlin/Getty Images

Something invisible holds the universe intact. It outweighs everything you can see—every star, every gas cloud, every galaxy—by a factor of five. We call it dark matter, and for decades, the standard, simple assumption has been that it does exactly one thing: pull.

That is, we have viewed dark matter as involving no pushing, no collisions, no chemistry—just gravity, acting in silence to hold together the cosmos.

That assumption is looking increasingly shaky.

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Three recent preprint papers that arrived within weeks of one another probe the possibility that dark matter is not a mute backdrop but an active participant in cosmic physics. Instead of just imparting a feeling through its gravitational pull, it may touch things via other interactions. Instead of being blandly inert, its properties may change depending on location. And instead of following a rather limited range of possibilities (because astronomers thought they ruled out other options long ago), dark matter may have a much richer set of manifestations than previously suspected.

None of these papers delivers a detection; we are very much still in the dark about dark matter. But together, they may redraw what we’re actually looking for.

Dark Matter That Collides

Let’s start with the most basic heresy: dark matter and ordinary matter may actually collide.

Ordinary matter—the protons, neutrons and electrons that make up everything you've ever touched—is governed by forces that dark matter supposedly ignores. But “supposedly” is doing a lot of work in that sentence. We have observational hints that dark matter has minimal, if any, nongravitational interactions with normal matter—the Bullet Cluster is an iconic example—but no experiment has ever directly confirmed that dark matter is purely gravitational. The assumption of inertness is a simplification we adopted because it made the models tractable. Whether it’s true is a separate question.

Connor Hainje and Glennys R. Farrar, both at New York University, decided to take that question seriously. Their new simulation method models notional dark matter interactions with particles called baryons—that is, mainly protons and neutrons—in and around a Milky Way–scale galaxy. They specifically looked at the regime in which dark matter particles would be comparable in mass to, or lighter than, the protons and neutrons they’d be scattering against. That’s a regime where the non-gravitational physics gets interesting—and where previous simulations had little to say.

The result is striking. In standard simulations, a galaxy’s visible matter—gas, dust, stars — sits frozen inside a much larger “halo” of dark matter, like a bug encased in amber. The halo is assumed to be immutable. The two don’t really talk.

But Hainje and Farrar’s simulation opens a communications channel. Just dialing up the rate of dark matter–baryon interactions reshapes the halo from the inside out, redistributing mass in the galaxy’s core in under a billion years. A billion years sounds like a long time, but in galactic terms, it’s a coffee break. And that redistribution matters: it brings the predicted dark matter density at a galaxy’s center into much better agreement with what telescopes actually see, easing a long-standing headache called the “core-cusp problem.”

Lies and Statistics

Here’s another unsettling possibility: some of the constraints we’ve placed on dark matter interactions may be a little premature.

The cosmic microwave background (CMB), which is the faint afterglow from the big bang, is our most sensitive probe of the conditions when the cosmos was just a few hundred thousand years old. If dark matter was scattering against regular matter in those early moments, it would have left a signature: subtle distortions in the CMB’s temperature and polarization pattern. From 2009 to 2013, the European Space Agency’s Planck satellite mapped those patterns with extraordinary precision, producing what remains a canonical dataset for analyzing the CMB. Physicists have used this Planck data to set upper limits on dark matter–proton scattering, and those limits look tight—too tight, possibly.

Maria C. Straight of the University of Texas at Austin and her colleagues found a p