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.
箱の中にボールがある。
There is a ball inside the box.
私たちの中に殺人者がいる。
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.
Aの中にBがある。
There is B within A.
Aの中にもBはある。
B exists also/even within A.
も = even/also, は = topic/contrast marker
Where A is:
科学的に正しいと考えられていること
things that are considered as scientifically correct
And B is:
正しくないこと
things that are incorrect
Finally, these are enclosed by 私は~と思う using quotative-と:
私は「(科学的に正しいと考えられていること)の中にも(正しくないこと)はある」と思う。
I think that there are incorrect things even among things that are considered as scientifically true.