Learn Dutch: Volgens deze theorie moet dat de conclusie zijn. - According to this theory, that must be the conclusion.

It expresses a logical deduction. Just like in English, moeten (must) is not only used for rules and obligations, but also to state that something is logically certain based on evidence. Here, the evidence is the theory.

Why is the verb moet placed before the subject dat?

Because of the verb-second (V2) rule. The sentence starts with the phrase Volgens deze theorie (According to this theory). Since this phrase takes up the first position, the verb moet must take the second position, pushing the subject dat to the third position.

Why do we use dat to mean "that" when conclusie is a de-word?

When identifying what something is using the verb zijn (to be), Dutch always uses the neutral pronoun dat for "that" or dit for "this", regardless of the gender of the noun that follows. Even though it is de conclusie, we still use the neutral dat.

Why is it deze theorie and not dit theorie?

Because theorie is a de-word (de theorie). For all de-words, the word for "this" is deze. You only use dit for het-words.