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