Ralga was a distant planet that was home to an advanced race that chose not to venture beyond the stars
Around 8,000 BC the Ralgans eventually evolved to the point where they no longer needed their physical bodies and were on the verge of ascending to a higher state of being, similar to a level that some of the First Ones had previously achieved. The Soul Hunters, sensing the passing of the Ralgans' physical forms, mis-interpreted it as a mass extinction and sent fleets of ships to collect the souls of the beings on Ralga before they would "die".
The souls were placed within a large spherical vessel, where it was stored within a Whisper Gallery until its discovery by Robert Bryson in 2263 on an expedition to a dead world. The vessel was brought to Babylon 5, where the truth about what happened on Ralga was discovered.