I was always wondering why almost every single planet in the galaxy has some amount of Dust (apart from it being used as currency, ofc). I know there are many ruins and so on, but it's still to much, right? Were the Endless seeding this thing across the galaxy?

Look I haven't paid the maid in eons! Its not my fault there's so much damn Dust everywhere. Do you know ho much Galactic Maids charges per planet? Its insane! 

The galaxy was full of Dust long before ES1 or 2 started. It's why the Harmony cannot leave.

Dust is a self replicating, sometimes inteligent swarm of nano-machines that can do or build almost anything.