The Dark Aeons are something that only appear in the Final Fantasy X: International version of the game I believe, as I have played the Final Fantasy X that was released in the United States multiple times and have never been able to face any of the Dark Aeons unfortunately.

Dark Valefor - 800,000 HP
Return to Besaid Island, and at the entrance a summoner will summon Dark Valefor. Valefor will start the battle with energy ray, so use a weapon with first strike and switch with Yuna. Summon an aeon to take the hit. After that, Valefor will attack only one person at a time. Use aeons overdrives if you feel the need to, Magus Sisters and Animas overdrives are useful. Once you are done, the summoner will be chased away by the village people. Remember to use auto-phoenix and hastega on all.

Dark Ifrit - 1,400,000 HP
Return to the Al Bhed's Home to find this beast trying to claw its way out of a crater. Go back once and Explore the crater again. Ifrit will now jump out and a dark summoner will appear behind you. Ifrit's attacks are slightly more powerfull than Valefor's, and they may deal over 9,999 damage. He can easily kill 1 of your characters off with his attacks, but he has a disadvantage as his attacks can only target 1 person at a time. So use hastega at the start of battle. When his overdrive is full, use an aeon to block the attack.

Dark Shiva - 1,100,000 HP
Return to Macalania Temple to find the Guado waiting for you. They will have Dark Shiva attack you straight off. You must have maximum agility for this battle, as Shiva will probably kill you off at the start of you don't. Use hastega and aim a few times, and when she is about to use diamond dust cast auto-life or use an aeon to block. Besides that she is pretty weak because of her low HP, if you keep attacking and let auto phoenix heal you, Shiva should only be able to do Diamond Dust once before she is dead. After she is defeated, then Guado guards will start chasing you. You will have to fight a lot of them if you don't run away, but they eventually will stop coming once you've killed enough of them.

Dark Ixion - 1,200,000 HP
While traveling through the Thunder Plains, you will encounter a man, when he runs chase him and after a bit you will run into 2 summoners. One of the summoners will call Dark Ixion and the fight will begin. Ixion will counter-attack any normal attacks you do.

Dark Bahamut - 4,000,000 HP
For Tidus's Celestial weapon you need the Sun Crest which is in the room where you fought Yunalesca. If you opened the chest after fighting Yunalesca, then good. If you didn't open it, then now you have to beat Dark Bahamut in order to get it. Bahamut is strong aswell as has a lot of HP and moves very fast. Bahamut's favourite tactic is when his overdrive bar is full, use Impulse and then Mega Flare to finish off your party. Auto Life protects from Mega Flare and impulse will not to do much damage if your defence is 200+.

Dark Yojimbo - 1,600,000 HP
Go all the way to the back of the Cave Of The Stolen Fayth. Start to make your way out, and a summoner will come and summon Dark Yojimbo. Run up to Dark Yojimbo if you wish to fight him, if not, then use the teleport pad to escape. Yojimbo's Agility and Evasion is very high. At the start of the battle cast hastega, auto-life, and protect on every character. Protect is very usefull in this battle, as it halves all the damage done by Dark Yojimbo. His overdrive is Zanmato, which instantly kills - and auto life does not protect you from this. So use an aeon to block. After the battle is over, you'll have to beat Yojimbo 4 more times all the way out of the cave. You can use the save point to heal yourself and save but if you die or reload you will have to start again (saving can gain you some dark matters).