I honestly don't either. I'm open to the possibility, and all the sci-fi fi stories of exotic civilizations millions of miles away are great fun, but I cannot understand why anyone would be certain.
The “observable” universe is kinda tricky to explain, but I’ll try. The farthest things we can see are the things whose light took this long to reach us (ie: since creation / big bang). They are, of course, very, very far away.
Anything further out than that and the light hasn’t reached us yet, but it might some day. It is impossible for us to know if there is anything beyond the observable universe because no radiation (light or otherwise) could have reached us yet.