When a massive star dies and explodes, it releases not only light but also invisible lightweight particles. They fly out in all directions at nearly the speed of light. Only a small fraction reaches Earth, but each explosion sends its own clump. These clumps stretch along the way: fast particles overtake slow ones, and by the time they arrive, a clump can be spread over tens of thousands of years.
Previously, it was thought that there were so many such clumps from all explosions in the galaxy that they merged into a steady stream, like the noise of rain, where individual drops can't be heard. The authors of the new study tested this numerically by reproducing the history of explosions in the Milky Way over billions of years. It turned out that over short observation times (years or decades), each clump contributes only to a very narrow range of energies. So the resulting signal on Earth is not smooth but bumpy, like a road surface after many patches.
Such an uneven stream changes the expected signal in detectors. The scientists built a tool to calculate this 'bumpy' stream for any lightweight particles. Applying it to two candidates, axion-like particles and fermionic dark matter, they obtained weaker constraints than before. This means such particles could exist in a wider range of parameters than previously thought.
A stretched clump of particles from a single explosion can be tens of thousands of years wide — longer than human civilization has existed.
In Stanislaw Lem's novel 'Solaris', the ocean sends signals that are hard to distinguish from random noise — similarly, here the signal from distant explosions is masked as background irregularities.