Space
Although there is no up or down in space, galaxies happen to be flat due to their rotation. Thus, as long as you're traveling within galaxies, it's still relatively easy to differentiate between horizontal movement and vertical movement.
Although there is no up or down in space, galaxies happen to be flat due to their rotation. Thus, as long as you're traveling within galaxies, it's still relatively easy to differentiate between horizontal movement and vertical movement.