Sentence improvement — noun clause as subject: keep the structure 'Why + clause + is not known'; the sentence 'Why the dinosaurs died out is not known' is already precise and formal.

Given sentence
Why the dinosaurs died out is not known.

Concept/Approach
A wh- clause ('Why the dinosaurs died out') can function as a subject. This impersonal, academic pattern is standard: 'Why … is not known.'

Option analysis
'it is not known'/'the reason is not known' → would require rephrasing the entire sentence and are less concise.'that is not known' → vague pronoun reference.

Final Answer
No improvement