So I've had this headcanon for a while now about stargate and finally have found the time/motivation to write it out
In Stargate Universe (I know this one isn't as widely loved as the first 2 series, but hear me out) it is established that the chair interface on the ship is a precursor to the chair we see in Antarctica and Atlantis. It is further established that the reason the Ancients sent out the Destiny was because of a recording in the background radiation of the universe. Furthermore, between The Ark of Truth and the beginning of SG-1 season 9 (Avalon) we know the Ancients/Alterans left thier home galaxy because of the Ori, and when they got to Earth and the Milky Way they "built many astria porta" or Stargates.
We also know the Ancients were one among the 4 Races who communicated with each other. The biggest one of these being the Asgard. And we know they live in a different galaxy from us Othala. Now I myself have always wondered how they encountered the Asgard when we have no evidence that they--for lack of a better word--colonozied the Asgard home galaxy. Even with concerted efforts to find other advanced life in the universe it'd be unlikely to happen by chance alone (even with advanced technology).
All of these tidbits may seem random and unrelated. But here's my headcanon on how they all tie together:
The Alterans were traveling to the Milky Way is how they got the recording of the background noise and they decided they needed to research the universe itself. So after settling on Earth they built the Destiny and its seed ships and sent them out (tangentially this could also explain why SGC can't find naquadah on Earth--the Alterans used it all in creating the stargates on these ships).
Now obviously sending out the ships they researched the Milky Way first and knew where to put stargates in this galaxy.
Furthermore, and more to the point, this would explain how the Ancients encountered the other 3 Races and established communication with them. And, again as an aside, this would explain how they knew about the Pegasus galaxy and that they could move there when they were experiencing the disease that nearly wiped out life in the Milky Way (Window of Oportunity, Frozen, and Reckoning Parts 1 and 2).
