The phrase "big mystery" refers to something that is unknown or not easily understood and has a significant level of complexity or intrigue surrounding it. Full definition
And there's an even bigger mystery underlying this biochemical web: why do we even get allergies at all? (discovermagazine.com)
One of the current big mysteries facing modern physics is the question of why everything seems to be made of one kind of matter, when there are two kinds. (businessinsider.com)
The recreation entails platforming and fixing puzzles and general bigger mysteries concerning the recreation, the world, and the top of time. (skilledgeekz.com)