Formal:
* Obscure: This word implies something is hidden or unknown, making it difficult to find or discover.
* Unheard-of: This is the most literal translation, suggesting something is completely unknown or unprecedented.
* Unremarkable: This implies something is ordinary and not noteworthy, making it unlikely to be talked about or remembered.
* Forgotten: This suggests something was once known but is now lost to memory.
* Unrecorded: This implies something was never documented or documented poorly, making it difficult to find information about it.
Informal:
* Lost in the shuffle: This suggests something was overlooked or forgotten amidst other things.
* Under the radar: This implies something is unnoticed or hidden from public attention.
* Off the grid: This suggests something is completely disconnected from the mainstream or conventional.
* No one's heard of it: This is a very direct and informal way to say something is unknown.
Other options:
* Neglected: This implies something is ignored or forgotten about.
* Overlooked: This implies something was missed or not considered important.
* Unimportant: This implies something is not worth mentioning or remembering.
* Unremarked upon: This suggests something went unnoticed or was not commented on.
The best word for you will depend on the specific context and the desired meaning.