Quote (Kviki)
It seems that after the engine renders the galaxy as the skybox, the skybox gets shaded so you can see the shadows at the edges of the cube.
No.
Skybox can be disabled in main.cfg:
TRMaxSkyBoxes 0
This will free 24 Mb of VRAM and remove seams, but will lead to significant drop of performance.
Setting TRMaxSkyBoxes 2 will enable skybox for nebulae (it is disabled by default), but takes another 24 Mb.