Lore question about the Dark Angels and The Fallen.

I'm new to 40k but, from what I understand, in the current era the Dark Angels and their successor chapters have been hunting the fallen for over 10k years now. I know that is slowly changing due to the Lion's approach to the matter but that's besides the point.

What I don't understand is: how come The Fallen are still alive after 10k years? That's too much, even for Astartes, right?