When a brandnew sub opens on another Lemmy instance, your instance doesn't know about it, thus you can't search for it from your own instance.

TIL how to "tell" my instance that this remote sub exists.

• Go to "Communities" in the Lemmy Web Interface
• Search for !community@instance.something. For example !todayilearned@lemmy.ml
• Now do another search for "todayilearned"
• Done. The sub is now searchable for everyone on your instance and you can subscribe to it.