Half the universe was missing... until now

Half the universe’s ordinary matter—baryons made of protons and neutrons—was long thought to be “missing” because telescopes and other observations only account for about half of the expected amount. The shortfall isn’t dark matter or dark energy; it’s normal matter that’s hard to detect because it sits between galaxies in a diffuse, ionized state. Recent measurements using fast radio bursts now line up with the missing-baryon predictions, putting the total baryonic matter at roughly 5% of the universe and showing that about half of those baryons reside in the warm-hot intergalactic medium (WHIM).

The expected baryon fraction comes from the early universe’s chemistry. In the first minutes after the Big Bang, the universe cooled enough for protons and neutrons to fuse, but only after a brief bottleneck: deuterium (a proton–neutron nucleus) had to form first. Once deuterium became stable against being immediately smashed apart, it rapidly fused into helium-4. This “freeze-out” locked in element abundances—about 75% hydrogen and 25% helium by mass. Deuterium is especially important because it’s stable and isn’t produced in large quantities by later astrophysical processes. By counting deuterium relative to hydrogen—about 26 deuterium nuclei per million hydrogen nuclei—and combining that with measurements of the cosmic microwave background radiation (the Big Bang’s afterglow), cosmologists infer that baryonic matter should make up about 5% of the universe.

In the late 1990s, astronomers attempted a census: adding up baryons in stars, planets, gas, dust, and other luminous or inferable structures. That inventory came up short, finding only about 2.5% of the universe in baryons—roughly half of what early-universe physics predicts. One part of the solution is straightforward: much ordinary matter isn’t glowing brightly. Neutral hydrogen can be traced using distant quasars as backlights. Their spectra show a “Lyman-alpha forest” of absorption lines—tiny notches caused by neutral hydrogen clouds along the line of sight. Summing those absorbers nearly doubles the baryon tally, bringing it close to 50% of the expected total.

The remaining baryons were predicted by simulations to be spread through intergalactic filaments at extremely low densities (about 1–10 particles per cubic meter) and at temperatures between roughly 100,000 and 10 million Kelvin. At those temperatures the gas is ionized, so it doesn’t produce the same absorption signatures and instead interacts mainly with high-energy ultraviolet and low-energy X-rays—making it notoriously difficult to detect.

A new approach uses radio waves instead of X-rays. Lightning-like dispersion in Earth’s magnetosphere can turn a pulse into a “whistler,” and the same physics applies in space: free electrons slow lower-frequency radio waves more than higher-frequency ones. Fast radio bursts (FRBs)—millisecond-long, extremely bright pulses discovered in 2007—provide the needed backlight. By measuring how much an FRB’s signal is dispersed and comparing the dispersion measure to the redshift of its host galaxy, researchers can estimate how many ionized electrons—and therefore how many ionized baryons—lie between Earth and the source. A recent Nature paper reported that this method recovers the missing baryons, finding a total baryonic fraction near 5% and placing about half of the baryons in the WHIM. The result largely vindicates decades-old simulations, while also underscoring how inefficiently the early universe’s ordinary matter ends up concentrated in stars and galaxies.