Dark Age of Technology
The Dark Age of Technology (or just the Age of Technology) is a period of history dominated by the human civilisation that flourished before the Imperium of Man. It takes place shortly after the discovery of the Warp drive and the Navigator gene.
Thanks to Warp travel and the Standard Template Constructor, humanity is able to spread far and wide, settling millions of planets and driving out the xenos before them. The result is something of a golden age for humanity, with technological wonders (beyond even the Adeptus Mechanicus's ability to recreate) being constructed.
The Age of Technology ends because humanity becomes dependent on their technology and on Warp travel. They lose the former with the rebellion of the Men of Iron, and they lose the latter as the increasing incidence of psykers and Eldar partying too hard causes Warp storms, Enslaver plagues, and close encounters of the daemonic kind on settlements and spacecraft (think "Event Horizon"). With humanity disrupted by these losses, the aliens that they had managed to put down come back with a vengeance, and humanity falls into what is later called the Age of Strife.
The "Dark" in "Dark Age of Technology" comes because, after the Age of Strife, the wars of the Great Crusade and Horus Heresy, and then the long millennia of the Age of the Imperium, almost no records remain of this age -- it is "dark" because it is almost entirely unknown, and the closest thing we can know about how Humanity lived during this Age was the Interex that was more advanced than the Imperium but was unfortunately wiped out by Erebus. It is also called a "Dark Age" because it was a spiritual dark age compared to the "glorious modern age" because humanity worshiped their own creations in pride, rather than the God-Emperor of Mankind. The irony of this is lost on every human in the setting. The only remnants are some of the technological wonders of the age, called "archeotechnology" or "archeotech," which the Adeptus Mechanicus furiously seeks. Some, like the STC, could be the Imperium's salvation; others, like the Men of Iron, are best left alone (or better yet, destroyed). Good luck getting a tech-priest to not poke around the possibly-dangerous ancient technology, though.
Although sources differ and generally argue, most point to human technology in this time being roughly equivalent to (if not surpassing) that of the pre-Slaanesh Eldar. Highlights of the few hints we've been offered include still-pristine warships with bridges made of solid light, what seem to be time-weapons, and a cloud of sentient nanomachines that kill you by making your blood explode. Dark Age humanity wasn't nothin' to fuck with.