hazard a guess

anyone's guess

A mystery; something unknown or unpredictable.

by guess or by gosh

at random

guess what

Used to introduce a surprising outcome or one that the hearer is not expected to try to guess.

you'll never guess

I have something very surprising to say; you won't be able to guess what I'm about to say.

your guess is as good as mine

I don't know; I have no idea.