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'..

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'..