Man, I have come to hate Dark Matters...
Not only are they required for pretty much every late game synthesis, but to make it worse they don't drop, and require Mythril to make (meaning you have to synthesize a lot of those too, farming Mystery Goo, etc.)
My advice, if you have not gotten to Final Rest yet, you will be DOING YOURSELF A HUGE FAVOR if you farm Mythril at that really really long heartless fight in End of the World. You can't do it after you've beaten the room though, unfortunately.
To get Mythril to drop, you need to be at End of the World and use the Bambi summon while getting it to the third gauge. Needless to say, this is impossible if you've already gotten through the swarm of heartless. To farm Mythril, leave the room before you finish and it will start over when you come back.