Under the Graphics Options, any Background Detail setting lower than "Super High" causes the display of the outline of a square. If I imagine that my ship is at the centre of a cube then just two opposite faces of the cube have just the vertices(?) showing. I can only see one square at a time and the view of the square changes as I change the direction of my ship.

(I thought it was some sort of visual aid for orientating in space, but none of the screen shots I looked at showed it... and it was getting on my nerves as it spoilt the cool views!)