A rather elusive species of aliens (the theory about their prehuman origin is almost completely disregarded) that not only achieved the pinnacle of technology... but also remained there for a significant amount of time, despite being in general decline for most of its history. It is also a species that seemed to have some activity in the star sectors surrounding Earth that are believed to be aware of existence of humanity.
Aurums are elusive because of their apparent mastery of technology and - especially - a spacetime manipulation. It is believed that most of their ruins are yet to be discovered, with all of them being a treasure trove of advanced technology that suffered nearly no decline due to age and the fact that many of them seem to suffer a devastating assaults during the last years of their civilization.
This is made even worse because of the fact that they seemed to be obsessed with hiding their settlements in a way that makes them extremely hard to discover. Even on a colonized planets their installations are sometimes discovered when their cloaking technology suffers malfunction.
Anatomy & Morphology
Completely unknown. There are no objects of art in their cities that could show how they looked, and their computer systems were never breached - in fact even their language remains a complete mystery. Because of that next to nothing is known about them. One can safely assume that they were humanoids and of similar size to humans.
Civilization and Culture
Average Technological Level
Absolutely masterful level of space manipulation. Entire spatial technology used by current humans is based solely on Aurum technology, in a vastly weakened version. That is, regardless of that, barely understood and mostly replicated without understanding.
Extremely advanced electronics, with their protections never succesfully defeated. Aurum technology is usable only in a level that is allowed without getting through security (in other words, if its unprotected), otherwise its unusable. And even then it has to be thoroughly tested to make sure what button causes what, and God have mercy if one of them is a self destruction button. They are rare, but spatial implosion is certainly not nice.
Two main periods of Aurum history can be marked, dubbed the Imperium period and the Diaspora period. The former one lasted from 5000 BC to 50 AD (approximately). The latter lasted from 50 AD to 1750 AD. Once again, approximately, since the dating isn't precisely consistent. There is a significant period of strife and changes in the middle, and Aurum Imperium seems to have locally lasted until 200 AD.
At an apex of the Imperium period the Aurums controlled area including Machine Cluster, Screaming Triangle, and significant part of Human Space, from Perseus Arm to Sagittarius Arm. With its western borders more or less overlaping with current Outer Colonies/Plants borders. They seemed to be pushed out of Screaming Triangle by Screamers around 500 BC, but the reason for that retreat is unknown - judging from Aurum strength and technological prowess they should easily triumph.
Fall of Aurum Imperium was caused by... something. Most of their cities seems to be suddenly abandoned, most of them completely destroyed. Either internal strife or an outside attack of unknown forces is suspected. The destruction seems to have started in western Machine Cluster and quickly spread throughout the whole Aurum space.
All Aurum Diaspora period seems to be overshadowed by a terrible fear of something stalking the surviving Aurums. All of their cities are spread far and wide but also very thin, with cloaking technology used to hide them as good as possible. Most of them has no signs of damage, but is nevertheless abandoned. Aurums made no attempts to face the unknown stalker (or at least no signs of major space battles conducted by aurums of Diaspora period were found), or to expand again, seemingly content with dying out.