You need wings to discovery to Break HP Limit, not dark matter. You can get about 8 by bribing Great Malboro for 1,280,000, bribing Malboro for 540,000 (x4), or beat Shinryu for 2 of them.
You'll need 30 of them to customize that ability on your armor.
I was planning on doing that for Auron only because he's the one whose Celestial weapon is more powerful when he's weak but I might do it for another character. I already have Break HP Limit on one of Yuna's armor, but I forgot what I beat to get it.