不得而知

unknown / unable to find out

no; not so / (bound form) not; un-

must not / may not / not to be allowed / cannot

to obtain / to get / to gain / to catch (a disease) / proper / suitable / proud / contented / to allow / to permit / ready / finished

structural particle: used after a verb (or adjective as main verb), linking it to following phrase indicating effect, degree, possibility etc

to have to / must / ought to / to need to

and / as well as / and so / but (not) / yet (not) / (indicates causal relation) / (indicates change of state) / (indicates contrast)

to know / to be aware