After a nearly twenty-year long game of cosmic hide-and-seek, astronomers using ESA’s XMM-Newton space observatory have finally found evidence of hot, diffuse gas permeating the cosmos, closing a puzzling gap in the overall budget of ‘normal’ matter in the Universe.

While the mysterious dark matter and dark energy make up about 25 and 70 percent of our cosmos respectively, the ordinary matter that makes up everything we see – from stars and galaxies to planets and people – amounts to only about five percent.

But even this five percent turns out to be quite hard to track down.

The total amount of ordinary matter, which astronomers refer to as baryons, can be estimated from observations of the Cosmic Microwave Background, which is the most ancient light in the history of the Universe, dating back to only about 380 000 years after the Big Bang.

Observations of very distant galaxies allow astronomers to follow the evolution of this matter throughout the Universe’s first couple billions of years. After that, however, more than half of it seemed to have gone missing.

“The missing baryons represent one of the biggest mysteries in modern astrophysics,” explains Fabrizio Nicastro, lead author of the paper presenting a solution to the mystery, published today in Nature.

“We know this matter must be out there, we see it in the early Universe, but then we can no longer get hold of it. Where did it go?”

Counting the population of stars in galaxies across the Universe, plus the interstellar gas that permeates galaxies – the raw material to create stars – only gets as far as a mere ten percent of all ordinary matter. Adding up the hot, diffuse gas in the haloes that encompass galaxies and the even hotter gas that fills galaxy clusters, which are the largest cosmic structures held together by gravity, raises the inventory to less than twenty percent.

This is not surprising: stars, galaxies and galaxy clusters form in the densest knots of the cosmic web, the filamentary distribution of both dark and ordinary matter that extends throughout the Universe. While these sites are dense, they are also rare, so not the best spots to look for the majority of cosmic matter.

Astronomers suspected that the ‘missing’ baryons must be lurking in the ubiquitous filaments of this cosmic web, where matter is however less dense and therefore more challenging to observe. Using different techniques over the years, they were able to locate a good chunk of this intergalactic material – mainly its cool and warm components – bringing up the total budget to a respectable 60 percent, but leaving the overall mystery still unsolved.