The Dark Forest

The Dark Forest theory gained a foothold in Earth thinking in the early 21st century, the idea that the reason they hadn't made first contact yet was that everyone else was hiding from... Something.

When, in the 23rd century, Earth did get a message, which was interpreted as "Shut up before it's too late", it obviously got a big boost in popularity.

But a hundred years later, when there was enough social pressure to force exploration crafts, Humanity discovered that the message wasn't so much a warning as an attempt to stop us expanding.

Because the fear was of us.