The Lost: The Hegira resulted in scattering of many people and ships, and not all went the same way as the Alliance, or fell behind. The Lost wander the galaxy, although rarely, a vessel finds its way to the Alliance. Rumors of Lost nations, some even of collaborators with the Shadow or something even darker, are unconfirmed, as few have the courage to venture deep into Shadow-held space. Most Lost are given up as dead, and, perhaps, they are.
I'm gonna expand on this, I was thinking of small corrupt groups helping the Shadow in espionage missions. What do you think?