There are quite a few in the Fediverse. Try the search bar, type in a topic, and give it a moment. (Sometimes it’s a bit slow to retrieve.). New communities are constantly being created, too, so check again in a few hours. It might be redundant to create one on Beehaw when there are already so many.

Edit: searching “UFO” gave me a long list of results, and several communities about UFOs, and I’m sure there will be more tomorrow. You can subscribe to one or all, and see them as soon as you log in. Set your front page to “subscribed” and you’re rolling!