Your example makes it sounds like it's "obtainium." Call it obtainium.
I can't think of a phrase that trips off the tongue, but non-metadata could be called "representational data" or "content data" or even "contextual data" as its interpretation is invariable dependent on the metadata.
If you want a generic term for things you pass to a function, you could say "additional parameters". Otherwise, you should start thinking about what this data is, rather than what it isn't.
P.S. This sounds exactly like the sort of problem where the obvious answer comes to you fifteen minutes after you step away from the computer.
I don't really love this answer, but it's different from the others so far, so how about...
To create Object Ω, call function ω and supply metadata; or if you know what you're doing, supply Explicit Initialization Parameters
As others have noted, it depends on context. In messaging, payload is commonly used.