those would be known affectionately as the brain boyz. they created the orks to work for them and imprinted them with certain "smartz" to make up different classes of orks. then something happened to them, no one knows, possibly too smart for their britches, overthrown, or even a crazy idea that the snotlings or grots are what used to be the brain boyz.
There are also plenty of texts here and there about an ancient race that once dominated the galaxy; even before the day of the Eldar's rule.
Evidence can be found within' the Jericho Reach Maw Warp Gate. Ordo Xenos has investigated it numerous times and is unable to trace it's origins from any known race. The Ordo believes it has been dormant for tens of millennia. So it has to be much older because then you put into account the time it was being used.
This Warp Gate is linked to another on the complete other side of the galaxy, and a warship may travel through and reach it's destination within' seconds, as compared a journey that would take years through the warp.
It is a dark and turbulent journey through the void of space. Any ship that is "less than sturdy" will come through either badly damage or in pieces.