At the center of the Milky Way galaxy is a dormant, supermassive black hole. But a new study suggests that the galaxy's core had once been ablaze six million years ago.

Current measurements of the Milky Way indicate that the galaxy weighs about 1 to 2 trillion times as much as the Sun. An invisible dark matter forms about five-sixths of the galaxy, while the remaining one-sixth is normal matter, which is made up of 150 to 300 billion solar masses.

However, astronomers counted up all the stars, gas and dust that could be seen and only came up with 65 billion solar masses. The rest of the normal matter, which is made of neutrons, protons and electrons, has been missing.

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

"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," Nicastro added.

The researchers were searching for the missing mass when they discovered evidence of galactic shock waves from an explosion in the Milky Way's core. According to the study, the center of the Milky Way was once active, with a tremendous amount of energy being swallowed by the black hole, during the time the first human ancestors -- the hominins -- walked the Earth.

However, about six million years later, the explosion of the energy due to the feeding of the Milky Way's black hole, which had gone into hibernation. The event sent shock waves that crossed 20,000 light-years of space, which, according to the researchers, can still be seen today.

Using measurements of X-ray absorption and computer models, the researchers had calculated the amount of normal matter that had been there before and how it was distributed. However, they could not match the observations with a smooth, uniform distribution of gas. Instead, they discovered a "bubble" at the center of the Milky Way, extending two-thirds of the way to Earth.

The researchers found that the bubble would have required a great amount of energy. They concluded that the super-hot, million-degree gas from the exploding quasar caused by the galaxy's active phase, which lasted 4 to 8 million years, could account for the up to 130 billion solar masses of material. This means that the missing matter had been "too hot to be seen."

According to the researchers, the proposed next-generation mission known as X-ray Surveyor, could be able to map out the bubble by observing fainter sources, and see finer details to gather more information about the missing mass.

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