Dust Wars, there's not one single definitive source but there are many in-game sources via quests, hero and planet bios, as well as background knowledge from other Endless games that you have to piece together. 

Basically after the Endless hunted down the Lost to near extinction (in order to harvest them for the magic-like substance known as Dust, that they were comprised of), and the two main factions of the Endless, known as Virtuals and Concretes, had a falling out and in a galaxy-wide civil war they sought to eliminate the other faction (think of it like if there was a nuclear holocaust on earth), both factions died out as a result with only a few select individuals and enclaves (canonically many of them are Virtuals) left over. 

The Endless aren't extinct per se, but they're like numbering in the hundreds or, dare I say, thousands, throughout the entire galaxy. However, their civilization is all but gone. 

I can provide a more detailed explanation but that's all you really need to know unless you really, really want to get into the lore.

wow, i can't thank you enough for that, i'm actually playing alot to the game to try to earn the lore. seeing that the lore of this game attract people make me feel good ^-^

There's a couple extra details I think I rember but I'm really not sure I haven't just imagined them... So instead for more endless information there's always this wiki page for the old Edless Space I found although I can't garantee for the information's accuracy or relevance to the ES2 canon.

https://endlessspace.fandom.com/wiki/Endless