For the first time, scientists have directly measured how much ordinary matter is inside cosmic voids — the Universe's most rarefied regions. They used fast radio bursts: their dispersion measure (signal delay) reveals the number of free electrons along the line of sight, regardless of the gas's temperature and phase. By overlaying 3,455 rays onto 1,288 voids, they found a significant electron deficit: baryon density inside voids is about 60% lower than the cosmic average. Combined with the Sunyaev–Zeldovich effect, they also estimated the gas temperature — around a million degrees, pointing to a thin, hot plasma. This approach, akin to an ultrasound scan of the cosmos, will soon, with new telescopes, allow us to build a 3D map of matter distribution.
The Universe is structured like a sponge: there are dense regions packed with galaxies and gas, and giant pores—cosmic voids. These voids occupy most of space, but their contents have remained a mystery because matter there barely reveals itself.
Now astronomers have 'felt' the voids using fast radio bursts—super-powerful pulses from distant galaxies. As these pulses travel through the pores, they slow down slightly when they encounter stray electrons—tiny charged particles lingering in the gas. It's like water losing speed as it flows around sponge fibers. By measuring the delay in thousands of such bursts, scientists have weighed the invisible gas for the first time, mostly hydrogen.
Paradox: this gas is heated to a million degrees, but due to its incredibly low density, it barely transfers heat. If you were inside a void, you'd freeze, not burn. Studying these voids is crucial: they let us investigate dark energy and the properties of nearly elusive neutrinos, refining models of the expansion of the Universe.
🎯 If the Milky Way were at the center of such a void, the night sky would be absolutely black—no galaxies at all, only a few faint stars.