Much of (what/which) scientists know about dinosaurs has been recently discovered.
The phenomenon of (what/which) are known as corporate networks has also attracted attention.
And yes, the answer is what. But why?
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Both of the clauses beginning with what are noun clauses. You can tell they're noun clauses because they're both the object of the preposition of.
Noun or complement clauses can function like nouns -- as subject, direct object, or prepositional object.
There are four types of complement clauses in English, and this is the type called an embedded question (or headless relative -- they're not that different) complement. (the others are Infinitive, Gerund, and tensed That-clause.)
Embedded questions are just regular Wh-questions, but they have three peculiarities that mark them as subordinate clauses:
It's the third peculiarity that's responsible; the normal distinction between open what and closed which is simply not available in embedded questions, just like the usual future sense of will is unavailable in if-clauses. There's a reason.
Which already has a role as a relative pronoun (and you can't use what as a relative pronoun). Relative pronouns are Adjective clauses, while Embedded Questions are Noun clauses. The purpose of an introductory marker like what or that or which is to indicate -- before it's parsed -- what kind of clause is coming up.
If which already marks adjective clauses, it's confusing to have another which that marks noun clauses. So we don't. What marks noun clauses and which doesn't, while which marks adjective clauses and what doesn't. That's all, really. It's kind of like opposing metal to colour in heraldry -- it improves clarity and avoids confusion in the signal.
One difference between the usage of "What" and "Which" is the difference between Noun Clauses and Adjective (Relative) Clauses.
The same confusion occurs when a student makes a sentence like:
The information WHAT scientists know about dinosaurs is limited. = X
This sentence should be:
WHAT scientists know about dinosaurs is limited. = Noun Clause
The information THAT scientists know about dinosaurs is limited. = Adjective Clause
Similarly, "Which" will become possible in your sentence inside an Adjective Clause. For example,
There is an existing body of knowledge about dinosaurs, much of WHICH has recently been discovered.
People are flocking to build what are known as corporate networks, the phenomenon of WHICH has attracted a lot of attention.