"unknown purpose" is correct and usable in written English.
You can use it when you are referring to something that is not explicitly stated or not known. For example: "The document had an unknown purpose, so we were not able to determine its meaning."
Aforementioned Whisper brings random strangers together for an unknown purpose.
They are there, sitting like small squiggles on a startling sonata, with an unknown Purpose.
There was a TV, VCR, DVD player, cable box, karaoke player for his songbird sister, some converter with an unknown purpose -- all linked by various cables.
Designed by the legendary firm of McKim, Mead & White, board officials said the headquarters was originally three buildings -- an Elks lodge, a courthouse and a third of unknown purpose -- that were built in the 1920's for $6 million.
In Wallinger's own 2011 Shadow Walker, there is a sense of unknown purpose, the shadow walking as though on a gym machine with the paving slabs sliding away beneath him.
If a gene with a previously unknown purpose appears active in a part of the brain responsible for mood, for example, then it becomes a target for manic depression and other mood disorders, Dr. Boguski said.
Nicky goes on to search for a pumpkin that would best suit his unknown purposes.
It is the strangely conspiratorial truth of the surveillance society we inhabit that there are unknown entities gathering our data for unknown purposes.
Then he went back again into the world of pumpkins, looking for the one that would best suit his unknown purposes.
Found not guilty of complicity in industrial-scale abuse of privacy and destruction of lives, the only conclusion is that she is the most incompetent executive ever to be re-employed, after recklessly paying out mountainous expenses for unknown purposes.
"I wouldn't want NASA to turn itself upside down to go there, for unknown purposes.
