mystery

big, great

The disappearance of the ancient treasure remains a big mystery that archaeologists are still trying to solve.

little, minor, small

Why he never answers his phone remains a small mystery to his friends.

complete, total

The new student remained a complete mystery to his classmates throughout the entire year.

whole

The detective finally solved the whole mystery after months of investigating the strange disappearance.

certain

There was a certain mystery about why he always left the party early.

central

The central mystery of the disappearance remained unsolved for many years.

deep, profound

The ancient temple held a profound mystery that archaeologists spent decades trying to solve.

murder

The detective spent months trying to solve the murder mystery that had shocked the entire town.

be, present (sb with), remain

The reason why my keys disappeared last week still remains a mystery.

have, hold

The old lighthouse has always held a mystery that locals were reluctant to explain.

be cloaked in, be shrouded in, be veiled in

The magician's disappearing act remained cloaked in mystery, leaving the audience completely puzzled.

lose

Once you understand how magic tricks work, they lose their mystery.

clear up, explain, resolve, solve, uncover, unlock, unravel

The detective finally unraveled the mystery of who had stolen the valuable painting from the museum.

shed/throw light on

The old letter finally threw light on the mystery of why grandma left town so suddenly.

explore, fathom, grapple with, penetrate, understand

Scientists have spent decades trying to fathom the mystery of why cats purr.

ponder (on)

He spent the evening pondering on the mystery of why his keys had vanished from the table.

deepen

The confusion deepened when nobody could explain where all the missing documents had gone.

surround sth

The strange disappearance of the old painting continues to surround the museum with mystery.

man, woman

The police are still searching for the mystery woman who left the anonymous letter at the station.

~ about

Nobody could solve the mystery about why the old house had been abandoned for so many years.

~ as to

There is still a mystery as to why the old factory suddenly closed down last year.

~ to

I couldn't understand why he left the country so suddenly—it remains a mystery to everyone.

an air/aura of mystery

The old mansion on the hill has always had an aura of mystery about it.

something of a mystery, take the mystery out of sth

Why some people enjoy horror films remains something of a mystery to me.