If you were to look into the past and find something, but then look to the present to discover it missing, chances are you would be confused. After all, things do not just go missing, but that is what appeared to happen for a great deal of mass in the Universe. Now researchers using the Chandra X-Ray Observatory, XMM-Newton, and Suzaku satellite may have found where this mass is hiding and it is a lot closer than many expected.
Looking into deep space lets one look back in time, and far enough back, astronomers can see there was so much mass in the Universe in the past. Peering around the Local Group though, it seems some of the mass disappeared, but the new data suggests it is actually in a low density but very hot and massive halo around galaxies. This halo is at least 600,000 light years across (the Milky Way is approximately 120,000 ltyrs across) and contains at least 10 billion solar masses of gas, but could be as high as 60 billion solar masses (the Milky Way is believe to be between 1 and 1.5 trillion solar masses). With such a low density, it has been hard to detect, and would be almost impossible to detect around another galaxy. Despite that though, it is still between 100,000 and 1,000,000 Kelvin, which is considerably hotter than the Sun.
Researchers have long known there is a great deal of mass between galaxies, but this halo of hot gas is much greater than expected, especially as it could be larger than current measurements indicate. At least we now have a better idea of where that mass went to.