Today, the supermassive black hole at the center of the Milky Way is rather quiet. New research suggests its relative dormancy is recent.

According to a new study published this week in the Astrophysical Journal, the Milky Way's black hole marked its transition to hibernation with a bang some 6 million years ago. The ripples of the high-energy shockwaves expelled by the explosion can still be seen today.

Scientists with the Harvard-Smithsonian Center for Astrophysics found evidence of the shockwaves while looking for the Milky Way's missing matter.

Some five-sixths of the galaxy is made of up dark matter. The rest is normal matter -- gas, stars, dust. But when astronomers add up all visible matter in the Milky Way, there's less than there should be. Between 85 and 235 billion solar masses worth of material is missing.

Researchers believe that missing material exists as a thin shroud of gas spread throughout the galaxy.

"We played a cosmic game of hide-and-seek. And we asked ourselves, where could the missing mass be hiding?" scientist Fabrizio Nicastro, a research associate at CfA and an astrophysicist at the Italian National Institute of Astrophysics, explained in a news 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."

By measuring the background X-rays absorbed by the fog, researchers were able to measure the amount and distribution of the gas throughout the Milky Way.

Their findings suggest the gas was blown outward by an explosion emanating from the quasar at the center of the Milky Way 6 million years ago. The shockwaves created an expanding bubble of gasless space, leaving the supermassive black hole without fresh gas to eat.

A collection of 6 million-year-old stars at the center of the galaxy corroborate the timeline, as the stars formed from the material that once fed the hungry black hole.

"The different lines of evidence all tie together very well," concluded Smithsonian scientists and study co-author Martin Elvis.

While the gas bubble doesn't completely solve the mystery of the Milky Way's missing mass, it moves astronomers closer to understanding the galaxy's unique composition.