On Earth, echoes are produced when sound waves bounce around like pinballs. In space, echoes are produced when light does the bouncing.

Just as sounds can echo, so, too, can cosmic light. But instead of ricocheting off damp cavern walls, light traveling through the universe bounces off soft, dusty clouds.

Sometimes, this happens after an explosive event such a supernova. On Earth, most of the light we’d see from one of these exploding stars would have come directly here. But supernovas explode in three dimensions, sending light in all directions. Not all of that light is aimed toward Earth. If the geometry is right, some of the light zooming away from Earth might run into a cloud and end up being redirected