The phrase is rather **ambigous**, so replacing it really depends on what you want to say:

- Your example means _'everyone knows'_. 
- But __usually__ one means _'Nobody knows'_, e.g. : _'God/Heaven knows when we'll meet again'_. 

For meaning one: use. _'Surely'_ or _'Everyone knows'_. 

For meaning two: use _'Who knows'_..