Up to 80 percent of the total mass of the Milky Way that's not dark matter is missing and scientists now say they know where it is.

Measurements show the Milky Way galaxy weighs about 1-2 trillion times as much as our Sun. About five-sixths of that mass is in the form of invisible and mysterious dark matter. Scientists estimate dark matter comprises up to 27 percent of the mass and energy in the observable universe.

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The remaining one-sixth of our galaxy's mass -- or 150 to 300 billion solar masses -- is normal matter. Oddly, when astronomers counted all the visible stars, gas and dust, they only found about 65 billion solar masses.

The question is where the rest of ordinary matter made of neutrons, protons and electrons went to. The answer: a fog.

"We played a cosmic game of hide-and-seek. And we asked ourselves, where could the missing mass be hiding?" said stufy lead author Fabrizio Nicastro, a research associate at the Harvard-Smithsonian Center for Astrophysics (CfA) and astrophysicist at the Italian National Institute of Astrophysics (INAF).

"We analyzed archival X-ray observations from the XMM-Newton spacecraft and found that the missing mass is in the form of a million-degree gaseous fog permeating our galaxy. That fog absorbs X-rays from more distant background sources."

Astronomers used the amount of absorption to calculate how much normal matter there is, and how it was distributed. They applied computer models but found they couldn't match the observations with a smooth, uniform distribution of gas.

Instead, they found a "bubble" in the center of our galaxy that extends two-thirds of the way to Earth.

Clearing out that bubble required a tremendous amount of energy. That energy, the authors surmise, came from the feeding black hole. While some infalling gas was swallowed by the black hole, other gas was pumped out at speeds of 2 million miles per hour (1,000 km/sec).

Six million years later, the shock wave created by that phase of activity has crossed 20,000 light years of space. Meanwhile, the black hole has run out of nearby food and gone into hibernation.

This timeline is corroborated by the presence of six million year-old stars near the galactic center. Those stars formed from some of the same material that once flowed toward the black hole.

"The different lines of evidence all tie together very well," said Smithsonian co-author Martin Elvis (CfA). "This active phase lasted for 4 to 8 million years, which is reasonable for a quasar."

The observations and associated computer models also show the hot, million-degree gas can account for up to 130 billion solar masses of material. Thus, it just might explain where all of the galaxy's missing matter was hiding: it was too hot to be seen.

More answers might come from the proposed next-generation space mission known as X-ray Surveyor. It will map the bubble by observing fainter sources, and see finer detail to tease out more information about the elusive missing mass.

The European Space Agency's Athena X-ray Observatory, planned for launch in 2028, offers similar promise.

TagsMilky Way, dark matter, solar masse, Harvard-Smithsonian Center for Astrophysics, Italian National Institute of Astrophysics, Black Hole, X-ray Surveyor