c. 1940s (?), as described in Geoffrey Pullum’s The Great Eskimo Vocabulary Hoax.
Snowclones are not always simply “journalistic clichés,” as Mark points out in “Snowclones are the dark matter of journalism.” Sometimes they do indeed reflect a lack of thoughtfulness on the part of the writer, and sometimes we get a glimpse into “a little conceptual universe, bringing along with it a metaphorical framework that structures the surrounding chunk of discourse” when we see one, as in the case of “X is the dark matter of Z”:
If X is the dark matter of Y, then X is crucial to Y, is even the biggest part of Y, but it is not directly visible, and must be inferred because of the strong effects it has on visible things.
In the original citation, [Instapundit Glenn] Reynolds goes on to say that “I have a few readers who function as virtual stringers, sending me several links throughout the day. Professional journalists sometimes send me links to articles or topics they can’t get assigned to write about, in hopes that I might get the story more attention.” By prefacing this with the dark matter business, he’s positioning his own experiences with emailed leads as characteristic of a universal phenomenon, suggesting that weblog publication is the visible manifestation of underlying social networks that operate via hidden email connections. This is a pretty efficient use of five little words.
So any individual instance of a snowclone may or may not have both of these things going on. At minimum, “they’re … a reflection of just how wide the vocabulary of a fluent speaker of English (or whatever language they occur in) is, because they’re memorized chunks much like other lexical items or idioms” (as The Tensor reminded me), and sometimes they tell something about their user’s world knowledge, and the knowledge shared between the user and his listener/reader.