Missing baryon problem

Settled in 2018 by Fabrizio Nicastro and colleagues (X-ray detection of warm-hot intergalactic medium along a quasar sightline).

By the late 1990s, precise measurements of light-element abundances (Big Bang nucleosynthesis) and the cosmic microwave background gave a confident prediction for the total amount of ordinary matter in the universe, but direct observational censuses of stars, galaxies, and cold gas in the low-redshift universe consistently came up roughly 30-40% short, since the predicted gas is diffuse and mostly ionized, making it nearly invisible in typical surveys. In 2018, Nicastro and colleagues used 18 days of Chandra X-ray Observatory time on a distant quasar to detect X-ray absorption from highly ionized oxygen in warm-hot intergalactic gas at the right density and temperature to account for the missing baryons, with follow-up observations supporting the result and largely closing the two-decade-old puzzle.

Nearly half of all the ordinary matter the Big Bang should have made was hiding in plain sight the whole time, diffused into gas so hot and thin it was almost transparent, and it took an 18-day stare at a single distant quasar to finally see it.

Explanations that circulated

• That the missing baryons were hiding inside undetected dark, cold gas clouds within galaxies — the resolution instead placed them in diffuse, very hot (10^5-10^7 K), tenuous intergalactic gas (the warm-hot intergalactic medium) threading the cosmic web between galaxies

References

• Observations of the missing baryons in the warm-hot intergalactic medium, F. Nicastro, Y. Krongold, J. Mathur, M. Elvis, et al., Nature, 2018