We know that the center of the Milky Way is presently silent and is occupied by a supermassive black hole. However, researchers from the Harvard-Smithsonian Center for Astrophysics believe it wasn’t always like this, and the center used to be very active 6 million years ago when the first hominins emerged on Earth.

Before the black hole went into hibernation, it emitted energy so massive that some gas was pumped out at 1,000 kilometers per second or 2 million miles per hour. According to co-author Martin Elvis of CfA, the former active phase lasted between 4 and 8 million years.

In fact, the scientists even say that evidence of the shock wave still persists today when they found that a wispy orange bubble has crossed 20,000 light years of space or from the center of the galaxy to two-thirds of the way to our planet.

This bubble or gaseous fog could explain why there is an inconsistency in the current estimations of the Milky Way’s mass. Experts say that the galaxy weighs up to 1 to 2 trillion solar masses, five-sixths of which is in the form of dark matter while one-sixth is in the form of normal matter.

However, normal matter’s mass, which equals 150 to 300 billion solar masses, seems questionable when all of the stars, gas, dust and other objects made of electrons, neutrons and protons only total up to 65 billion solar masses.

“We played a cosmic game of hide and seek. And we asked ourselves, where could the missing mass be hiding?” points out the study’s 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.”

The hidden matter was apparently too hot to be seen before. The activity that caused the bubble is also supported by the presence of stars as young as 6 million years located near the center of the Milky Way.

Further details could also be gathered with the help of the X-ray Surveyor. This space mission could help uncover other hidden bubbles.