The cosmic census comes up short
For decades, astronomers faced a deep accounting error. When they tallied up all the ordinary, visible matter in the universe (stars, gas, dust, planets) the numbers didn't add up. Cosmological models based on the afterglow of the Big Bang, the Cosmic Microwave Background, predicted that about 5% of the universe's total mass and energy is "baryonic" matter. This is the familiar material made of protons and neutrons that constitutes everything we can directly see. Yet, observations of galaxies and galaxy clusters could only find about half of the expected amount. The rest was simply missing.
This "Missing Baryon Problem" was a long-standing puzzle. The matter had to exist somewhere, but it was invisible to conventional telescopes. The leading theory was that the missing atoms were spread thinly throughout the vast voids between galaxies, forming a tenuous, web-like structure. This network, known as the Warm-Hot Intergalactic Medium (WHIM), was predicted to be a plasma with temperatures ranging from 100,000 to 10 million Kelvin. However, its density is incredibly low, with perhaps only a few particles per cubic meter, making it almost impossible to detect directly.
A remote telescope provides the answer
The solution was from a remote patch of Western Australian desert, at the Murchison Radio-astronomy Observatory (MRO). This site hosts the Australian Square Kilometre Array Pathfinder (ASKAP), a radio telescope with a uniquely wide field of view. Scientists used ASKAP to hunt for Fast Radio Bursts (FRBs), which are mysterious and intensely powerful blasts of radio waves from distant galaxies that last only a few milliseconds.
These bursts are cosmic probes. As an FRB travels across billions of light-years, its signal gets slightly delayed and smeared out by the particles it encounters. By measuring the amount of this dispersion, astronomers can calculate the total number of free electrons along the line of sight from the burst's origin to Earth. The important step was using ASKAP's precision to pinpoint the exact host galaxy of an FRB. Knowing the distance to the galaxy allowed astronomers to subtract the matter within our own galaxy and the host galaxy, isolating the contribution from the intergalactic space between them.
In a landmark 2020 paper in Nature, researchers led by Jean-Pierre Macquart announced they had used this technique on a set of localized FRBs. Their calculations precisely accounted for the missing matter, confirming it resides in the diffuse, ionized gas of the WHIM. The decades-long search was over, and the cosmic books were finally balanced, thanks to a combination of mysterious cosmic signals and a purpose-built observatory in one of Earth's most radio-quiet locations.