It's only taken me 19 years to realise that some items on site are darkmatter and some are dark matter.

Darkmatter examples:

Dark Matter examples:

Is this deliberate or an oversight?

I could be wrong, but I believe Dark Matter (w/ space) = specifically refers to the minions (e.g., the treats, the sign, etc). Darkmatter (no space) = the color (e.g., Darkmatter ___, Darkmatter pets, etc).

has it right, according to this thread Consistency Issues in Items - 2.0 (post 3):

Thank you both ^_^

I'm going to go ahead and lock this up since it's not a bug. :)