I have been hearing all of this stuff about dark aeons and stuff and I have no idea what anyone is talking about. What are dark aeons and where do I get them???????? I am very very very very very confused.

Special monsters in the International and PAL version of FFX... Don't be looking for them in the US version.

Basic gist of it: They are bonus monsters that you can fight in Final Fantasy X (European or International Edition). However, they serve no purpose in the story or game except for harder battles.

In FFX PAL/ International version Dark Aeons are stronger versions of regular Aeons available after gaining the airship and before you fight Sin. They are very strong, ie even Dark Valefor has strength of 120, but are optional. In the international/ PAL version, 2 Dark Aeons block areas. Dark Valefor blocks Besaid, and Dark Shiva blocks Macalania Teple. One advantage of fighting them is that they may drop armour with the 'Ribbon' ability. This ability protects you from all status effects and is only available in the PAL/ International versions.
They are extremely hard to defeat and even if you got the International version, you would need very high statitistic levels and Break HP/ Damage limit to even hope to beat them.

Thanks for helpin guys I didn't know they were only in those versions so thanks for tellin me. Much appreciated!!!!

You're not missing much, anyways. :P

well, when I met the dark aeon Valefor for the first time, I was dead in 5sec =P
they are just ultra hard and they are blocking the way (wanted to go to Besaid to collect the remaining Al Bhed Volumes)

Ha ha, the US version doesn't have it :P You guys are missing a good challenge.