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.
as written by the authors arXiv:2605.01994
The pristine underdense patches of the Universe, cosmic voids, are powerful cosmological laboratories, uniquely sensitive to dark energy, modified gravity, and neutrino masses, yet their baryonic content remains uncharacterized. We present the first observational constraint on baryon underdensity in void interiors, exploiting the dispersion measures (DMs) of Fast Radio Bursts (FRBs) as tracers of the free electron column, independent of gas phase, temperature, and metallicity. By stacking 3,455 sightlines from CHIME/FRB on 1,288 SDSS BOSS voids over redshifts $$0.2 < z < 0.7$$, we measure a DM deficit toward void centers at $$3.2\sigma$$ significance, establishing that diffuse baryons inhabit the emptiest corners of the cosmic web at a suppressed level. The measured signal amplitude is consistent with an effective Universe model built directly from the observed galaxy underdensity in these voids, and a baryonic model calibrated to the FRB DM-redshift relation ($$\alpha_v = 1.80 \pm 0.87$$). A uniform-density void model yields an electron density contrast of $$\delta_\mathrm{e,v} = -0.58 \pm 0.30$$, implying a $$\sim 60$$% underdensity of baryons in void interiors relative to the cosmic mean. Jointly interpreting our FRB measurement with existing stacks of the thermal Sunyaev-Zel'dovich effect on voids further constrains the mean void gas temperature to $$T_\mathrm{e} \lesssim (1.1 \pm 0.7) \times 10^6$$ K, pointing to a warm-hot diffuse phase, consistent with hydrodynamical simulation predictions. With forthcoming FRB (CHORD, DSA, SKA) and galaxy (DESI, LSST, Euclid, PFS-Subaru, SPHEREx, Roman) surveys, set to expand both samples by orders of magnitude, this approach opens a new window onto tomographic baryon mapping, with direct implications for feedback models governing gas expulsion into low-density environments, and for the use of cosmic voids to extract cosmological constraints.
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.