No one knows who built them, but judging by how they look, they were created by a humanoid species. Dating methods on derelict Imogen spacecraft have revealed that they were around when the Prae were in their prime.
They are fairly friendly, but prefer seclusion. They have an ambassador on the Empyrean, but he is rarely seen. The Unai Ambassador once said the following about him: "Nu a namrasu suye atwu luname di zae minde e banda amaru." Translation: "It is difficult to speak with someone who sees you as a child."
Their technology is unlike anything, but they refuse to share their knowledge. The only ones known to have Imogen technology are the Myrmidons.
Mass Effect's Geth and Reapers.
Battlestar Galactica's Cylons and Star Trek's Borg.