Fermi paradox

The Fermi Paradox is a big question in space science. It asks why we haven't seen any aliens, even though the universe is so big that it should be filled with them. For example, if Earth was the only planet in the universe, it would be really hard to believe. So, why haven't we seen or heard from aliens yet? Scientists think that either aliens don't exist, or they are very far away and we can't find them.