The Universe grows like a metropolis: galaxies skyscrapers are erected on an invisible scaffold of dark matter: its gravity pulling in construction material. Meanwhile, dark energy makes the city itself continuously expand. Previously, it was assumed the scaffold compacts at a steady rate, despite the expansion of space. But recent measurements show that in recent epochs, the pace of construction has slowed down.
Astronomers suspect the issue lies with the scaffold itself: it gradually decays, emitting a 'dust' of lightweight particles. This dust, like a headwind, presses on the remaining dark matter and hinders further building. The most surprising thing is that the slowdown didn't start right away, but only a few billion years ago, as if the construction material suddenly started losing strength. The model predicts a clear signal — a characteristic break in the growth chart of galaxies on certain scales. Projects Euclid and Rubin will soon be able to test this hypothesis.
Dark radiation interacts so weakly with ordinary matter that it passes through Earth almost unnoticed.