Chiss were scattered, their home being a frozen ball of ice.
NR discovered Csilla and in a generous effort to draw the families out terraformed Csilla.
Some families resettled.
Others didn't and formed some of the Chiss groups that have graced TRF.
NR fell.
Empire conquered Csilla.
Wrath Virus devestated the population.
The Empire's/Kaine's Project Insidion did the rest.
Now: Imperial Expeditionary Forces inhabits the planet with few Chiss remaining.
I would assume that there are other families elsewhere in the unknown regions doing their isolationist thing (at least that would be the easiest premise to bring a Chiss group back with).