It's so, so annoying when stuff gets mislaid, so it must be a great relief to astronomers to be able to say they've probably found the nearby universe's missing matter.
Based on observations from NASA's Chandra X-ray Observatory and ESA's XMM-Newton, they've discovered a vast reservoir of intergalactic gas about 400 million light years from Earth.
This, they say, is the strongest evidence yet that the missing matter in the nearby universe is located in an enormous web of hot, diffuse gas.
This missing matter – which is different from dark matter - is composed of common-or-garden baryons. Measurements of distant gas clouds and galaxies have given a good estimate of the amount of this normal matter present when the universe was only a few billion years old.
However, the much older, nearby universe seems to have only about half as much as predicted.
This latest work supports the suggestion that the missing stuff is mostly found in a web of hot, diffuse gas known as the Warm-Hot Intergalactic Medium (WHIM), and not down the back of the sofa at all.
Scientists think the WHIM is material left over after the formation of galaxies, which was later enriched by elements blown out of galaxies.
"Evidence for the WHIM is really difficult to find because this stuff is so diffuse and easy to see right through," says lead author Taotao Fang of the University of California.
To look for the WHIM, the researchers examined X-ray observations of a rapidly growing supermassive black hole known as an active galactic nucleus, or AGN. This AGN, which is about two billion light years away, generates immense amounts of X-ray light as it pulls matter inwards.
Lying along the line of sight to this AGN, ]about 400 million light years away, is the so-called Sculptor Wall. This enormous structure contains thousands of galaxies - and a fair amount of the WHIM if theoretical simulations are
If so, the WHIM in the wall should absorb some of the X-rays from the AGN - and this is exactly what the astronomers found.
"Having good detections of the WHIM with two different telescopes is really a big deal," said co-author David Buote, also from the University of California. "This gives us a lot of confidence that we have truly found this missing matter."