Prior to the start of the war with the Covenant, humanity had established colonies on around 800 separate worlds. Most human colonies were small, populated by well under a million people, while a few were very large, with a population in the hundreds of millions.
Though the Forerunners disappeared over 100,000 years ago, their legacy lives on in the form of their massive constructs.
ANCIENT HUMAN WORLDS
Before human civilization as we know even existed, ancient humanity was travelling the stars and building a galactic empire.