No, that’s not a bleach stain on the Milky Way. This object is called the “Magellanic Stream.” It was discovered in 1965 in the southern hemisphere of the Milky Way, covering at least 180 degrees of sky (it’s about 600,000 light-years in length). The mysterious structure is located about 180,000 light-years from the Milky Way. The gas moves at excessive speeds, and in a direction that doesn’t match patterns found within the Milky Way. As a result, it has been the source of much astronomical scrutiny.
However, the long search for the object that spawned this beast is over. This structure was found to originate from the Large and Small Magellanic Clouds, two satellite galaxies of the Milky Way. Through this new data, originating from Hubble’s Cosmic Origins Spectrograph, scientists were able to determine that the material was likely freed from the gravitational bondage of the two satellite galaxies some 2-billion years ago, when the two galaxies interacted with each other. Most of the material is thought to originate from the Small Magellanic Cloud because it’s smaller and would have felt a stronger pull from the bullying Large Magellanic Cloud.
Another mystery solved. Yay.