I searched 뭔 on this site, but I have no result at all.
Is it right?
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무엇은 is a logical guess since 뭐 is short for 무엇, but it is wrong.
The reason 뭔 doesn't mean 무엇은 is likely because 무엇은 is a phrase almost never used in practice. If you are asking something phrasing it like English "what is ...?", you'd say 무엇이 or its short form 뭐가, not 무엇은. This is because 은/는 either states some idea you have or present something in contrast with others, and 무엇 just cannot have either of these senses since it is word for the unknown. 무엇 fits 이/가, which states something that happens to be occurring or needed in reality. That is, you have a need to know "what" rather than presenting something from your knowledge or feelings.
In short, 무엇이 and 뭐가 are very common but 무엇은 is almost never heard, and 뭔 means 무슨 which is a determiner like "what", as in "what problem" (무슨 문제). 뭔 is very informal, though, so it is rarely used in texts.
To be precise,