In terms of knowledge:

* Unknown: This is the most direct opposite, meaning something that is not understood or familiar.

* Unfamiliar: Something that is new or strange.

* Mysterious: Something that is puzzling or difficult to understand.

* Hidden: Something that is concealed or kept secret.

In terms of recognition:

* Unrecognized: Something that is not acknowledged or identified.

* Anonymous: Something that is without a name or identity.

In terms of fame or notoriety:

* Obscure: Something that is not well-known or famous.

* Forgotten: Something that is no longer remembered.

The best opposite will depend on the specific situation and the intended meaning.