For years, astronomers were baffled by dark spots in the night sky. I mean, sure, most of the night sky is dark by nature, what with the sun being on the other side of the Earth. But as telescopes grew from basic to big, those studying the movements of the stars started to notice the same occasional phenomena.
Where there should be a star, there sometimes wasn’t. As they tracked the movements of the objects nearby, astronomers were sure that some massive body at the center of it all was affecting the pieces around it, but nothing showed up on their telescopes.