The Fallen used to be a race called the Eliksni. They were visited by the traveler before our time. When the darkness found the Eliksni, the traveler abandoned them, fleeing in fear. The traveler then found humanity. When the darkness eventually caught up with it again, instead of fleeing again, the traveler decided to stand it's ground and fight back against the darkness, fighting to protect humanity. The Fallen hate the human race for this reason. The traveler chose to protect us, but abandoned them. The Eliksni became obsessed with the traveler. Their civilization was decimated by the darkness, and in the wake of their own 'collapse' (which they call 'the whirlwind') they set out to find the traveler once more, hoping to bring salvation to their people. Their obsession with finding it, and their pilgrimage across the stars, has lead to the downfall of their race, eventually becoming the nomadic pirates we now know as The Fallen.

Actually I think that Rasputin sabotaged the traveler so that it couldn't flee. Anonpig or someone has a theory on it

That theory doesn't really relate to the lore I explained above. But if I was to comment on that theory, I would say it is highly unlikely. Sounds pretty out there.

But supposedly the traveler was going to flee from is but Rasputin crippled it so it couldn't flee and leave.

Hummm interesting

It makes sense. Rasputin dwells in the darkness to understand it. Rasputin disables traveler with darkness. Traveler can't flee. Traveler stays and helps us. Maybe Rasputin is actually working against us but we just aren't able to tell. What if Rasputin is only fending off other creatures of the darkness because he wants control over the traveler.

They did. That's why Captains and Servitors still possess some ability to Blink. I also believe that's why some Fallen worship Servitors. They are spheres much like the Traveler. The storytelling in this game is terrible, but the lore has no equal. It's fascinating to me.