What counts as baryonic matter?

All matter made of protons and neutrons: stars, planets, gas, dust, and diffuse intergalactic plasma. Electrons are included by convention.

Where is most of the baryonic matter?

Most is in the warm-hot intergalactic medium (WHIM) between galaxies. Only a small fraction is in stars and planets.

Primarily from Big Bang nucleosynthesis abundances (deuterium, helium) and the acoustic peaks of the cosmic microwave background.

Compute the baryon (ordinary matter) density parameter Omega_b from the baryon density and the critical density.

Baryon Density

kg/m³

Critical Density

kg/m³

Baryon Density Parameter

0.044351

Baryon Fraction (%)4.435 %

Baryon Density Parameter vs Baryon Density

Ordinary (baryonic) matter—protons, neutrons, electrons—makes up only a small fraction of the universe.

Omega_b = rho_b / rho_crit

Current measurements give Omega_b about 0.049, meaning only roughly 5% of the total energy budget is ordinary matter.

rho_b = 4.2e-28 kg/m³, rho_crit = 9.47e-27 kg/m³.