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its a thin aluminum cover covering up all the goodies underneath. there is probably just a little room underneath there. dont sweat
 
I'm not trying to find out and get paranoid about something else that could be potentially wrong with my MBP lol
 
Aluminium is light and flexible, the flex there is entirely normal. As stated, underneath the aluminium bottom cover is a small gap to allow airflow and aid in cooling.
 
I think you might want to get a psych eval.
Seriously. If you want no flex at all you need to get some HP toughbook and not a shiny toy that is built as thin as possible.
 
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