The late-stage Universe can be thought of as a crowd dispersing after a concert. In the standard cosmological model ΛCDM, people shuffle away quietly, not bothering each other. But what if thousands of concertgoers suddenly start gently nudging their neighbors? That's exactly how axions, prime candidates for dark matter, behave.

This resolves cosmology's greatest puzzle: observers near the stage notice the crowd thinning faster than those at the back see. Nearby measurements (from supernovae, Adam Riess's project) show 73, distant ones (from the cosmic microwave background) 67. The model proposed by Roberto Peccei and Helen Quinn explains the gap like this: axions occasionally collide, and these micro-nudges on galactic scales make space expand more briskly. A surprising fact: this invisible particle originally emerged from the theory of strong interactions, and now it reconciles warring measurements.