Humanity has always needed sailors. First there were the sailors of the oceans, then the pilots of the air, and then, in this story, the Tractonauts: navigators of cosmic routes that run somewhere between gravity, dark matter and mathematical intuition.
Not all of them can see those routes. They find their way through other senses. Some hear the tracks as music. Some feel them as a tension in space, the way an old injury feels the weather coming. They guide the Arks of humanity through the galaxy, and the old tale says that whoever has once heard the song of the cosmic tracks will never return to Earth.
The tale leaves it open whether this is a curse or a calling. I have left it open as well.