Visitors

The name given to extradimensional alien visiting traders that arrived in human space in 3204 and left shortly afterwards.
They exchanged advanced technological trade goods with a number of human polities.

They have not been seen since.
In the years following their appearance the Maldenite Democratic Foundation became very active in acquiring all of the artifacts left behind. It would seem that the Mald were very concerned about cultural contamination (or 'culture shock') from an advanced alien culture.