Rare things do not enter the galaxy, as has already been answered in a few forum posts this week. Only those that currently exist can be acquired by players; new ones are no longer added.
As for how they came about, it depends. Some were rewards for community games held a long time ago (that will not return), others were previously on the CP exchange but were replaced by what is on the CP exchange now.
Do actually all items listed as rare actually exist in the Combine at least once? Because I heard eg. that several players had an ARC-170, but I haven't heard of anyone that has a (full) Clone Trooper Set.