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Answering well-known fundamental questions is usually regarded as the majorgoal of science. Discovery of other unknown and fundamental questions is,however, even more important. Recognition that "we didn't know anything" is thebasic scientific driver for the next generation. Cosmology indeed enjoys suchan exciting epoch. What is the composition of our universe? This is one of thewell-known fundamental questions that philosophers, astronomers and physicistshave tried to answer for centuries. Around the end of the last century,cosmologists finally recognized that "We didn't know anything". Except foratoms that comprise slightly less than 5% of the universe, our universe isapparently dominated by unknown components; 23% is the known unknown (darkmatter), and 72% is the unknown unknown (dark energy). In the course ofanswering a known fundamental question, we have discovered an unknown, evenmore fundamental, question: "What is dark matter? What is dark energy?" Thereare a variety of realistic particle physics models for dark matter, and itsexperimental detection may be within reach. On the other hand, it is fair tosay that there is no widely accepted theoretical framework to describe thenature of dark energy. This is exactly why astronomical observations will playa key role in unveiling its nature. I will review our current understanding ofthe "dark sky", and then present on-going Japanese project, SuMIRe, to discovereven more unexpected questions.

Unknowns and unknown unknowns: from dark sky to dark matter and dark energy