Where do the bubbles outside the Milky Way come from?

Back in 2020, astronomers made a striking discovery in the first complete sky map from the eRosita X-ray telescope aboard the SRG observatory: a huge circular structure of hot gas below the Milky Way plane that occupies most of the southern sky. A similar structure in the northern sky, called the “North Polar Spur,” had been known for some time and was thought to have originated from an early supernova explosion. Taken together, the northern and southern structures instead both appear to emerge ...