Star Wars, unfortunately. I used to tell myself I'm just going to look up one thing about Darth Vader after watching, and three hours later I'm reading lore about someone whose entire contribution to the franchise was standing in the background for half a scene THEN the next thing I knew I was stuck in a rabbit hole of said franchise.

My second hyperfixation are sharks because they're unfairly misunderstood and, more importantly, very cool. Expand that to marine wildlife in general at this point... I could spend an embarrassing amount of time reading about deep-sea creatures and their crazy evolutionary tales.