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human
[Warhammer 40,000] What interactions occurred between the Eldar and Dark Age Era Humanity?

I've heard that at it's height, the Eldar empire was the pre-eminent power in the Galaxy. Similarly, Mankind was at its technological height during the Dark Age of Technology. If I'm reading things correctly, it seems like the Fall of the Eldar and the Age of Strife both occurred nearly simultaneously. Furthermore, it would seem that these two societies co-existed for nearly 10 Millenia.

My question is, what sort of interactions did they have. If Mankind reached it's zenith in terms of technology and spread at the same time as the Eldar were the Galaxy's most powerful society, how could they have co-existed as they did? Given that the current Imperium encompasses nearly the entire galaxy, and Dark Age humanity was apparently even more widespread, I don't see how the Eldar could ALSO have been controlling most of the galaxy.

Basically, both species were at their heights at the same time, and the current Imperium is big enough to control the majority of the galaxy right now. So how the hell could both of them have controlled more than 50% of the galaxy at the same time? Plus, whatever the relationship was, what do know of their interactions?
gpt
They didn't.

Mankind, at that point, had no single, unifying leader. They had technology which was closing on the Eldar, who weren't really advancing - having a webway, a post-scarcity society and an infinite supply of chocolate sauce and lubricant does that for you. Die? Fine! Just run straight into the warp and hang around for a few minutes until you can reincarnate. This caused warp storms, which began to become more and more frequent during the DAoT, a time when Man had no single leader. The Emperor was lurking on earth somewhere, but he hadn't really subjugated the world yet. Working on it, though. 

Anyway, while man puttered about, the Eldar had truly staggering amounts of sex and drugs and everything and being so massively psychically active, pumped all that excess into the warp. Travel became extremely difficult if you relied on the warp. And dangerous. Warp storms gathered more and more around the embryonic Slaanesh, until its birthscreams created the Eye of Terror, ended the Eldar (barring a few hardy dissidents who decided they would follow excess in their own way, the Exodites, or excess in denial, the Craftworlds, while claiming not to) and got rid of all the warp storms shrouding the galaxy.

This was the point when Big E said to himself "Now or never" and launched a tiny little galaxy-spanning crusade to find his lost kids and unite the galaxy under his iron-fisted rule.