The word that is missing in the given sentence is the preposition <em>in</em>. Prepositions are words used to link nouns, pronouns, or phrases to other words within a sentence and express the relationship between them.
The phrase <em>in the absence of (someone or something) / in (someone's or something's) absence</em> means <em>while someone/something is away</em>. The word <em>in </em>will always be used in this case.
