The Milky Way is surrounded by a massive halo of hot gas

An international team of astronomers has combined data from NASA's Chandra X-ray observatory, ESA's XMM-Newton space observatory and Japan's Suzaku satellite to suggest that our galaxy may be surrounded by a halo of hot gas extending in all directions for hundreds of thousands of light-years. 

The finding also offers clues as to why more than half of the ordinary matter in early galaxies has seemingly disappeared without leaving a trace.

Protons and neutrons are classified as "baryons," a type of subatomic particle that interacts strongly to form the nuclei of atoms. Taken together, baryons make up nearly all of the ordinary matter in our universe.

But if you were to tally the number of atoms in the universe, you'd find that something doesn't quite add up.

Astronomers have observed that entire galaxies seem to lose over half of their atoms compared to when they first formed.

All of this matter couldn't have simply disintegrated, so where has it gone? This decade-old question is known as the problem of the "missing baryons."

Now, a team of astronomers led by Dr. Anjali Gupta may have just found the answer, at least for our galaxy.

The baryons, says Gupta, haven't disappeared from the Milky Way. Rather, a mass of up to sixty billion suns – well in excess of the matter contained in the entire galactic disk – is spread out over a halo of hot gas stretching all around us for hundreds of thousands of light-years.

This gas is reaching temperatures in the millions of degrees, and with a density so low that, even if it were present in other galaxies, we would probably have no way of detecting it.

"With reasonable assumptions, our observations imply a huge reservoir of hot gas around the Milky Way," said co-author Smita Mathur of Ohio State University in Columbus.

"It may extend for a few hundred thousand light-years around the Milky Way or it may extend farther into the surrounding local group of galaxies. Either way, its mass appears to be very large."

A paper detailing the study was published on The Astrophysical Journal.