Impossible to know, especially being beyond the range of human experience or understanding: the unknowable mysteries of life. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "unknowable" in a sentence
"The characters, including Cromwell, remain unknowable, their emotions closely guarded; this works well for court intrigues, less so for fiction."
"I like how this method provides structure while allowing the story to shoot off in unknowable directions."
"Most decidedly, this is not artificial life (though I hasten to add that there is nothing mystical or long-term unknowable about components of living cells and organisms, including the eventual ability to tweak them)."