something is anyone / anybody's guess
Meaning: it is not possible for anyone to really know something.
He has a plan, but whether it will work or not is anyone’s guess
>> Ele tem um plano, mas se vai funcionar ou não, quem sabe?
The future of the country's economy is unknown
The future of the country's economy is a mystery
>> O futuro econômico do país é uma incógnita
REFERENCE: Cambridge Online Dictionary