Most of the universe is made up of mysterious “dark matter” and “dark energy”. The search for these often overshadows the fact that we still have not found a lot of the ordinary matter known to exist. A new study claims that the evidence for the location of this “missing matter” was detected 20 years ago by the early X-ray satellite ROSAT but missed in previous analyses.
Measurements of the early universe reveal how much matter of the form that should be visible to us (not dark matter or dark energy) was created in the Big Bang. This material should be made up of baryons, the category of particles that includes protons and neutrons. It could take the form of stars, planets, or clouds of gas.
Yet attempts to estimate the mass of galaxies and other matter we can see produce not much more than half of that. Astronomers concluded that large quantities of matter are lurking somewhere we were missing, inspiring searches to fill this gap. Earlier this year, evidence emerged that there is more material than we realized in the space between galaxies, which is affecting the transmission of fast radio bursts (FRBs).
However, a single paper is seldom the last word on a longstanding mystery, and a new study has reanalyzed old data to provide an alternative way of detecting the missing material's location.
Most of the universe's mass is thought to lie in enormous filaments, of which galaxies are just the densest clumps. Dr Nabila Aghanim of the Institut d'Astrophysique Spatiale stacked ROSAT2 images on top of each other in order to make visible the X-ray emissions that are too weak to pick up in a single image.
Having found evidence for diffuse X-rays in certain parts of the sky, Aghanim and co-authors report in Astronomy and Astrophysics that the locations match those of 15,000 known filaments more than 100 million light-years long. Galaxy clusters were masked out to confirm the presence of X-rays nearby. The paper attributes the X-rays to gas surrounding galaxies hot enough to produce emissions, but at such low density we have struggled to detect it.
Although the number of particles per cubic meter in these locations is so low it would seem like a vacuum for most purposes, space is, as Douglas Adams told us, “big. Really big”. Adams was talking about the distances inside galaxies, but outside of them there is even more volume – enough to hold a lot of matter, even at extraordinarily low densities.
How well the hot, X-ray emitting gas Aghanim has found matches up with the radio-distorting intergalactic material remains to be seen. The paper describes the results as consistent with estimates of gas density obtained from some other methods.