This ～の中に means "in ～", "inside ～", "among ～" or "within ～". 中 by itself is a noun that means "interior", "middle", "content", etc. It can be used with a group (of people, objects, ideas, etc), too.
There is a ball inside the box.
There is a murderer among us.
I suppose this is easy for you, but this sentence has several nested clauses, which seems to have confused you. Let's start from a simple sentence.
There is B within A.
B exists also/even within A.
も = even/also, は = topic/contrast marker
Where A is:
things that are considered as scientifically correct
And B is:
things that are incorrect
Finally, these are enclosed by 私は～と思う using quotative-と:
I think that there are incorrect things even among things that are considered as scientifically true.