This image, captured by the Hubble Space Telescope, depicts the Magellanic Stream: a long ribbon of gas which stretches nearly halfway around our galaxy. But it also solves a 40-year mystery of where the damn thing came from, too.

The image shows how the Large and Small Magellanic Clouds—two dwarf galaxies which orbit our very own Milky Way—are at the head of the gaseous stream. Since it was discovered in the 70s, scientists have wondered if the gas came from one or both of the clouds.

Now, Hubble observations have confirmed that the original stream came into being when gas was stripped from the Small Magellanic Cloud about 2 billion years ago. Then, much more recently, a second stream of gas from the Large Magellanic Cloud joined it. Case closed! [NASA]