An archipelago is a chain or cluster of islands. The name means "chief sea", derived from Greek words meaning "leader"-"sea". The word originated as the name for the Aegean Sea, which is the sea between modern Greece and Turkey. That area contains a large number of scattered islands. Archipelago later was used to describe just these islands in the Aegean Sea. Today the word describes a chain or cluster of islands, like the Hawaiian ones or the West Indies.
When I look at the map of the Aegean Sea and all these little islands, I also notice that most of the cities mentioned in the NT are also there, but on the land masses of modern Greece and Turkey.
Isaiah is the book in the Bible that mentions islands fourteen times. Through Isaiah's perspective, it seems like islands are "out there", but also matter and are very known to God and need the word, just like the mainland does.
There are the Rome and Jerusalem churches that everyone knows about. There are the Asia minor churches that many know about. There are also the achipelago churches, that are linked or chained together usually. Most people don't know about these "out there" churches. They all matter to God and God sees all of them together.