If you are someone that struggles with finding your keys or glasses, you're not alone. In fact, astronomers have it way worse: they can't find missing matter in the cosmos.
Most galaxies are surrounded by a lot of matter that we cannot see – so-called dark matter – but they are missing regular matter, which is expected to be in these galaxies. Some nearby galaxies have only one-third of the expected values, the Milky Way has just half the expected matter. Where is the rest?
"This has long been a mystery, and scientists have spent a lot of effort searching for this missing matter," lead author Jiangtao Li of the University of Michigan, said in a statement. "Why is it not in galaxies – or is it there, but we are just not seeing it? If it's not there, where is it? It is important we solve this puzzle, as it is one of the most uncertain parts of our models of both the early universe and of how galaxies form."
To answer these questions, an international team looked at six nearby galaxies. They suspected that matter might be found as very hot and low-density gas clouds in the halo of these galaxies. As reported in the Astrophysical Journal, they were able to find some of this missing matter there, but not enough. Almost three-quarters of the Milky Way's missing matter is still missing.
The missing matter is supposed to be extremely faint, so actually looking for it is challenging. To search for it, the team used the European Space Agency's XMM-Newton X-ray observatory. To capture the elusive signal they combined the data from the six observed galaxies creating a single galaxy observation with average properties.
"By doing this, the galaxy's signal becomes stronger and the X-ray background becomes better behaved," co-author Joel Bregman, also of the University of Michigan, added. "We were then able to see the X-ray emission to about three times further out than if observing a single galaxy, which made our extrapolation more accurate and reliable."
Although they didn’t find all the missing matter, the team has helped significantly in the hunt. And about the rest of the missing matter, astronomers believe that it is either in a different gas phase (hotter or colder), which would require a different instrument, or it is in a location of space (like further out from the galaxy) currently not being monitored
The missing matter is believed to have been ejected into intergalactic space by the action of supermassive black holes or by stars going supernova. Finding it will help us with the creation of more accurate models of galaxy evolution.