-I'm not entirely sure, but from a quick glanse at some tabs I got it to E major.
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"Ah yeah, ah yeah, ah, ah, ah. Ah yeah, ah yeah, ah, ah, ah." - Robert Plant
I would say it is in C# minor during the main part, since all of the chords in it work out (except for the G# major, but it is borrowed from the C# harmonic minor scale), and the outro is in Em.
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