Dark Matter????

[Masamune·1600]

The following is taken from a post I made in the Odd Items thread further down the FFX Forum page.

You will automatically receive 99 Dark Matters at the Monster Arena after capturing five of every Fiend, although you can also obtain the item as a rare drop from pretty much every Area Conquest, Species Conquest, and Original Creation at the Arena as well. You can also Bribe them from Land Worms inside Sin.

As CaptRory noted, however, you would be better off powering up the Celestial Weapons, as they have special attributes that improve their damage beyond normal Customized Break Damage Limit weapons. Information on the Celestial Weapons can be found in a thread stickied at the top of the FFX page.