Dark matter has always been described as an invisible substance that responds only to gravity. It neither emits nor absorbs light, nor does it interact with ordinary matter in any detectable way — except through its gravitational pull on galaxies and galaxy clusters. But a growing body of evidence suggests dark matter particles may interact with one another through a hidden force of their own, a 'dark sector' force that operates alongside gravity.
A new analysis published in August 2026 challenges the simplest versions of this idea. Using cosmological data from the Dark Energy Spectroscopic Instrument and the Planck satellite, researchers tested whether a dark force — one that acts only between dark matter particles — would make the universe look different from what we observe. The answer surprised them: the force exists, but it produces the opposite effect of what most models predicted.
Instead of clumping dark matter more tightly, as a self-attractive force would, the hidden force appears to push dark matter particles apart, suppressing the formation of small-scale structures. This is the opposite of what scientists expected from a simple attractive dark force. The finding rules out entire families of theoretical models and narrows the range of possible explanations for how dark matter behaves.
The result has implications beyond particle physics. If dark matter experiences a repulsive self-interaction, it would help explain why some dwarf galaxies are less dense and more spread out than simulations predict. It could also affect how galaxies form and evolve over cosmic time.
Knowledge takeaway: dark matter particles may interact through a hidden repulsive force that suppresses small-scale structure formation, ruling out many theoretical models and helping explain why dwarf galaxies are less dense than simulations predict.