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.
The hot intergalactic gas that is so hard to account for makes up about half of all ordinary matter in the universe — more than all the stars combined.
In the novel 'Darkness at the End of the Tunnel,' the decay of dark matter threatens to alter the very physics of space, making stars unstable.