This paper reports tighter limits on a possible small component of dark matter made of ultra‑light axions. Ultra‑light axions (ULAs) are hypothetical particles with extremely small mass (between 10⁻³² and 10⁻²⁴ electronvolts in this study). They act like dark matter in the average expansion of the universe but can slow the growth of structure on large scales because their quantum wavelength is very large. That slowdown leaves a characteristic, step‑like suppression in the clustering of matter that galaxy surveys and the Cosmic Microwave Background (CMB) can detect or rule out.

The authors combined a full‑shape analysis of galaxy clustering from the first data release of the Dark Energy Spectroscopic Instrument (DESI DR1) with CMB measurements from the Atacama Cosmology Telescope (ACT) and the Planck satellite. DESI DR1 includes nearly six million tracers of large‑scale structure and covers an effective cosmic volume of about 18 gigaparsecs cubed. The team used a modern theoretical framework called the Effective Field Theory of Large Scale Structure to model galaxy clustering and to separate the signal of a small axion component from other astrophysical effects.

Their main quantitative result is a strong upper limit on how large a fraction of the matter ULAs can make up, as a function of axion mass. For the lightest masses they study, the joint DESI+ACT+Planck analysis improves on CMB‑only limits by more than a factor of two and gives the most stringent bounds to date. For example, axions with mass around 10⁻²⁹ eV are limited to contribute at most about 0.3% of the total matter energy density. The DESI luminous red galaxy sample alone shows a mild preference for an axion fraction near mass 10⁻²⁶ eV, a hint similar to earlier notes from the Baryon Oscillation Spectroscopic Survey (BOSS), but that preference disappears when the CMB data are included.