Dark Age of Technology
The Dark Age of Technology (or just the Age of Technology) was a period of history before the Imperium of Man, when the human civilization flourished and dominated the stars. It took place shortly after the discovery of the Warp-drive and the Navigator gene.
Thanks to Warp travel and the Standard Template Constructs, humanity is able to spread it's reach far and wide, settling millions of planets and driving / pushing 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 and laws of physics being rewritten.
The Age of Technology ends because humanity becomes too dependent on 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"). While humanity is being disrupted by these setbacks, the aliens that were previously thought to be driven off, retaliate and come back for that sweet sweet vengeance, and so humanity falls a period that would later be 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 were more advanced than the Imperium, but were unfortunately wiped out by the Luna Wolves, manipulated into doing that by Erebus. It is also called a "Dark Age" because it was a spiritual dark age compared to the "glorious modern age of rationality", because humanity proudly worshiped their own creations, 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 searches for anywhere possible. 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.