In the first moments after the Big Bang, the universe was a hot soup of particles. Among them could have been neutrinos — very light and nearly elusive particles. Some of them might have been 'sterile': they barely interact with the rest of matter, like a quiet guest at a loud party. If such particles exist, they could make up dark matter — the invisible substance that holds galaxies together.

The authors of the new study considered how sterile neutrinos appeared in the early universe. Ordinary neutrinos, colliding with each other, could produce their sterile counterparts. These newborn particles flew off in different directions, and their motion influenced how clumps of ordinary matter later gathered. Such a trace can be seen in the distribution of galaxies and in how distant gas absorbs light.

Comparing predictions with data on the cosmic microwave background (light left over from the universe's youth) and on Lyman-alpha forests (absorption patterns in the spectra of distant quasars), the scientists obtained strict limits on the mass of sterile neutrinos. These limits are comparable to the best laboratory constraints but come from cosmology — the science of the universe as a whole.

This approach is like detective work: without seeing the particles themselves, we reconstruct their properties from the traces they left on a cosmic scale.