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An accountant tallies up all the factories, warehouses, and shops, but the balance doesn't add up — half the inventory seems to have evaporated. That's roughly how astronomers have spent decades searching for ordinary matter in the present-day Universe. Stars, gas in galaxies, plasma in clusters — all together, they account for only a fraction of what should be there according to the early epochs.

It was thought that the missing matter hides in the cosmic web — a network of tenuous gas filaments connecting galaxies. But this gas is too hot (a couple of million degrees) and too diffuse to glow in ordinary telescopes. The authors of the new work came up with a clever trick: they combined two completely different signals. The first is fast radio bursts (FRBs), short pulses that travel across the cosmos and get slightly delayed if electrons lie in their path. The second is a full-sky map of thermal emission built by the Planck satellite using the Sunyaev–Zeldovich effect: hot gas slightly warms the cosmic microwave background, leaving "shadows" on it.

The researchers took a catalog of radio bursts from the CHIME telescope and overlaid it on Planck's thermal map. To exclude massive galaxy clusters, where gas is abundant, they carefully cut them out, like using a stencil. The remaining signal showed a clear correlation: where the map is slightly warmer, radio bursts are delayed more. The probability of a chance coincidence is less than three-tenths of a percent. This is the first direct indication that diffuse gas in the web truly exists and makes up about 48% of all baryons in the local Universe.

And so the accountant's puzzle is solved. The missing atoms didn't vanish: they're just too transparent and too hot to spot individually. But when radio bursts pass through them and a thermal map is layered on top, the invisible becomes visible.