Someone one time had a cool way to make the universes work on a forum post I read once.
I know you Star Wars fans don't acknowledge midichlorians because you think space-magic is more plausible but hear me out and don't leave!!!
Midichlorians draw in warp energy!!!

Jedi = psykers. And it works for both lores!
I mean, how Chaos uses the warp and how Eldar use the warp is essentially the light and dark side of the force.
I like how a lot of the answers are "space marines roflstomp everything and you lose". Really? And you say that a normal star wars soldier couldn't stand up to space marines. Then by that logic the eldar, orks, nids, IG, and tau should've all been eradicated by the space marines already.