For those interested in peeking into 21st-century physics, dark matter and dark energy are appropriate topics. The science is still being sorted on these topics. I wrote question sets for episodes of Physics for the 21st Century devoted to dark matter and dark energy. NOVA Wonders produced a more recent episode devoted to both dark matter and dark energy. Much more slickly produced than the earlier Annenberg series, for what it's worth.
The universe is hiding something. In fact, it is hiding a lot. Everything we experience on Earth, the stars and galaxies we see in the cosmos—all the “normal” matter and energy that we understand—make up only 5% of the known universe. Find out how scientists are discovering new secrets about the history of the universe, and why they’re predicting a shocking future.