The universe is like a vast warehouse where a strict inventory was taken long ago. The cosmic microwave background is an ancient record of everything in stock 380,000 years after the big bang. According to it, the fraction of matter in the cosmic 'budget' (the parameter Omega_m) is known with high precision.
Now, billions of years later, astronomers are trying to recount everything, and they run into chaos. Some ordinary matter hides in the diffuse hot gas between galaxies, while dark matter clumps around them. The smallest clumps, like dusty boxes in a far corner, are nearly invisible. As a result, the total never quite matches the ancient record. The discrepancy could indicate that some dark matter has simply decayed into other particles over time.
New tools (spectroscopic sky surveys, next-generation X-ray telescopes, and maps of light bending by massive objects) will help bring order. They are like modern warehouse scanners: they will illuminate hidden gas, probe dark corners, and finally tell us whether today's balance matches yesterday's. If not, then something in the warehouse is indeed shrinking.
as written by the authors arXiv:2606.19837
An enduring signature of the decay of unstable dark matter constituents into other particles would manifest as a measurable discrepancy in the matter density parameter (Omega_m) between the recombination era (z ~ 1000) and the local Universe (z = 0). While precision measurements of the Cosmic Microwave Background tightly constrain the initial matter budget, evaluating this decay hypothesis requires an equally precise audit of the current epoch. We find that current local inventories of baryonic and dark matter are subject to systematic uncertainties - particularly in accounting for the warm-hot intergalactic medium, diffuse intra-cluster media, and the exact profiles of low-mass dark matter halos - rendering a definitive verdict on late-time dark matter decay currently hard to pin down. Furthermore, existing astrophysical bounds on cosmic rays, the diffuse gamma-ray background, and reionization history already heavily constrain potential decay channels during this epoch. However, next-generation observational technology is poised to resolve these local accounting gaps. Upcoming high-resolution spectroscopic surveys, next-decade X-ray missions, and advanced weak lensing campaigns will drastically reduce baryon and mass-mapping uncertainties, transforming the late-time matter audit into a cleaner, more definitive test.
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