<u>"After Darnell visited his aunt in Buffalo" functions as an introductory clause, helping us contextualize the information that comes after it in the sentence. Introductory clauses or phrases are commonly separated from the rest of the sentence by a comma, especially when they are long, just like the one we have here.</u> Therefore, the best way to punctuate it would be:
After Darnell visited his aunt in Buffalo, he couldn't stop talking about the Albright-Knox Art Gallery, home to one of the best modern art collections in the country.
