In English, it's actually very common to use "it" with no antecedent (check out the use of "it's" in this sentence!). We do this in set expressions, or to describe general states of being. Some examples:

It is raining
It's a sunny day
It's a shame that you couldn't come
It pays to learn grammar
It doesn't matter
Forget about it!
It isn't as bad as you think

In many of these examples, you can think of substituting "the situation" in for "it": "the situation isn't as bad as you think." (This doesn't really work for set idiomatic expressions like "it pays," though).

On the GMAT, you will rarely encounter "it" used as a general pronoun on SC. Almost always, "it" should have a specific antecedent. The GMAT will often turn something like "it is commonly believed..." into "scientists commonly believe..." in a correct answer. The first isn't grammatically incorrect, but the meaning is less clear.

It's not uncommon (see what I did there?) to see "it" used as a general pronoun without antecedent on RC or CR, though. In that case, don't really worry about finding the antecedent.

The example you gave was of an entire paragraph (which contained several other grammar errors), which you will never see on SC. The best way to study is from GMAT-like sources!