Nebula
A nebula is an astronomical phenomena that can be present in sectors. They usually appear as strange colored shapes in the skybox of a sector.
Solar systems in a sector with a nebula will have nebula clouds in them and different background color depending on the nebula. Small space rocks seen when moving in space will also appear larger inside a nebula.
Aside from the visual effects, there is nothing interesting about nebulas.
If the visual effects of a nebula is annoying, they can be disabled in the settings.