Learn Swedish: Studenten ska försvara avhandlingen, eftersom hon förklarar en ny teori däri. - The student will defend the thesis, since she explains a new theory therein.

It replaces the phrase i den (in it) and refers back to avhandlingen (the thesis). In formal or academic Swedish, you can combine där with a preposition like i to create däri (therein), rather than using a preposition plus a pronoun.

Could I just say i den instead of däri at the end of the sentence?

Yes, absolutely. Däri is highly formal and almost exclusively found in academic texts, legal documents, or older literature. In everyday speech and informal writing, you would naturally say eftersom hon förklarar en ny teori i den.

Why is däri placed at the very end of the clause?

Because it acts as an adverbial of place, telling us where the theory is explained. In Swedish word order, location adverbials normally go at the very end of the clause, which is exactly where the phrase i den would sit.

Why is ska used here for the future tense instead of kommer att?

The verb ska often implies a firm intention, an obligation, or a scheduled plan. A thesis defense is a highly planned and required event, making ska the most natural choice. You generally use kommer att for future predictions that do not involve personal intention.