Just wondering, would it ever be possible to finish a whole game of Endless Space 2, from start to finish, as the Umbrals without contacting any Major Factions, or letting them figure out you even exist?

I've attempted a run at it before, but my sanctuaries got spotted, reloading saves didn't seem to change much expect which faction happened to spot me first.

This might be an interesting challenge and wondered what you guys thought of it.

very interesting challenge. imo it is, but in lower difficulties

Very interesting indeed.  I don't think it is.  Though I managed to remain undetected for the first 2/3 of a game at normal difficulty and normal speed, but once the technology advances and the others reach detection level 3, remaining undetected becomes largely impossible.  Okay, perhaps impossible is an overstatement.  Let's say improbable.

It might be possible to relocate the UC home system to an unconnected node, well off the beaten path and try to hide there practically doing next to nothing for the entire game (no sanctuaries in other systems, no hacking others, no sending exploration vessels, etc.) except for developing the home system alone and see if it'd work but that would defeat the purpose of trying to win the game.

IMO it is possible if you rush a science victory in easy or sandbox, because due to the bonus you have for difficulty lvl you should be able to win before others reach detection lvl 3.