The phrase "well known" means widely recognized or familiar. Some antonyms for "well known" are:
Obscure: Something that is obscure is not widely known or understood. It is hidden or not prominent. For example, "The author's early works are rather obscure compared to his well-known later novels." Obscurity implies a lack of visibility and recognition.
Unknown: This is a straightforward opposite. If something is unknown, it has not been discovered or is not familiar to people. For instance, "The destination was an unknown place to most travelers, very different from well-known tourist spots." Unknown indicates a complete lack of knowledge or awareness.
Neglected: While not an exact antonym, neglected can imply that something has not received attention and therefore is not well known. For example, "The old building had been neglected for years and was not well known even among locals." Neglected suggests a state of being overlooked and not made prominent.
In conclusion, these antonyms for "well known" offer different ways of describing things that lack the recognition and familiarity that "well known" implies. Whether it's being obscure, unknown, or neglected, they stand in contrast to the widespread knowledge and recognition associated with being well known.