When you look for something in the place it should be but find nothing, it can be frustrating to continue the search, and we're not just talking about your car keys. For a long time, astronomers couldn't find a large portion of the matter in the universe. Now, however, they think they have finally found it all.
The matter that makes stars, planets, and all of us is referred to as baryons. Based on our model of the Big Bang, researchers estimated how much ordinary matter there should be in the universe, but when you compare it to all the known baryonic matter, about 30 to 40 percent appears to be missing. Researchers suspected that the gas was probably spread out between galaxies, but suspecting it and proving it are two very different things. Several studies have found indications of it and now new research, published in Nature, has enough evidence to confirm that the missing baryons have been found.
“After 18 years of continuous attempts from several different research groups worldwide, our observations have finally found the ordinary missing matter of the Universe,” lead author Dr Fabrizio Nicastro said in an Italian press release. “The matter we have found is exactly where and how much is expected in theory, so we can say that we have solved one of the biggest mysteries of modern astrophysics: the missing baryon problem.”
If this matter has been stretched between galaxies all along, why did it take so long to find? The issue is that this material is hot (millions of degrees) and it's spread so thinly that it almost doesn’t affect the light that passes through it. The main component of this is ionized hydrogen, which is extremely difficult to observe. This means astronomers had to get creative.
They used several satellites to observe a very powerful quasar, called 1ES 1553+113, for three weeks. This extensive observation allowed them to find a weak absorption signal. Something between the quasar and us was able to absorb some photons. The team confirmed the presence of oxygen with a temperature of about 1 million degrees Celsius (1.8 million degrees Fahrenheit).
The observations were performed in 2015 and 2017, and they found the same signal. The lack of change suggests that it is unlikely that this was material spewed out by the quasar and that it is actually the stuff spread across intergalactic space. Galaxies account for 10 percent of all the baryons, 60 percent sits in diffuse clouds between them, and the remaining 30 percent is this warm-hot intergalactic medium.
The team will now observe more quasars to confirm beyond any doubt that all the baryons have been accounted for.