[dappa]

The dark aeons will show up when you have acquired the airship for good.

Dark valefor - entrance to besaid village
Dark ifrit - where the destroyed al bhed base is
Dark Ixion - Thunder plains near the second save point
Dark shiva - entrance to macalania temple
Dark yojimbo - cavern of the stolen fayth right near yojimbos resting place
Dark Bahamut - Where you fought yunalesca
Dark Anima - replay the trials of gagazet with wakka then go to the start of gagazet where the ronsos live.
Dark Magus Sisters - The start of mushroom rock road.

You really need maximum stats to fight dark ixion onwards, the first two can be overcome by some really cool planning.

All the dark aeons have their own special auto abilities too