Dark Matter should be returned to gathering points.

[Catharsis]

With DM5 at 350gil at NPC, it's still cheaper (around 30% cheaper to repair with DM) <snip>

Except as I said, it's not pragmatic nor practical to enter a dungeon with your weapon or gear at 33% or under, since you can't exactly switch jobs and repair it on the fly (like you could before). This isn't so much an issue for trials, at least. This means that you'll sometimes likely repair damage which is under 350 gil.

More importantly @Orohin: seeing as the Active Help says that you can get dark matter from field activities and this isn't the case, there might not actually be a reason and this might be an oversight either with the Active Help or with the database devs, who may have forgotten to add it to the harvesting list (they might have confused it with Carbonized Matter, which is different). It's entirely possible, isn't it? Oversights do happen. Some confirmation on whether it was intentional or not would be appreciated.