If you have all the other aeons, there will be just this light, hazzy, purple-ish....energy barrier blocking the door, and by checking it the game will tell you that you needs some items. If you don't have all the other aeons, there will be this darker, thicker, red energy barrier blocking the door, and by checking it the game will just tell you that nothing is happening. And you only need to defeat Belgemine's Bahamut to get the item for the door. But you must have all the other aeons.