The Galactic Rim (or The Rim) was the outermost extent of the Milky Way galaxy. The older races, or First Ones, have travelled beyond the Rim into the void between galaxies.
The Galactic Rim is also used as a generic term that refers to the edge of known space within the galaxy itself; as new systems are being charted and explored all the time by the younger races, the idea of the Rim is an ever-changing concept.[1]
Before its destruction, the planet Z'ha'dum was located near the Galactic Rim.[2].
Background information[]
When talking about cast members that have passed away, series creator J. Michael Straczynski refers to them as having "passed beyond the Rim".