hyphenation - To make a noun to describe everything that is known well, would it be 'the well-known'?

For example, would there be a hyphen in the following sentence: "What exasperated him most was the well-known"? Or would it be: "What exasperated him most was the well known"? I know there may better/different ways to express the same sentiment ... But I'm interested in this particular usage. Does the hyphen belong there?

Thanks!