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