Why astrophysicists think there's a black hole in our solar system

In 2016, astrophysicists Konstantin Batygin and Mike Brown noticed unusual gravitational activity in the outer solar system. They speculated this activity was from a ninth planet situated in the Kuiper belt. But now, astrophysicists James Unwin and Jakub Scholtz propose Planet 9 isn't a planet but actually a primordial black hole.