Watch NOVA scienceNOW Season 3 Online

Dark Matter - Turns out most of the universe is held together by a mysterious, invisible substance. Of Mice and Memory - Mice placed in enriched environments can recover lost memories, giving hope to those who study Alzheimer's. Profile: Hany Farid - This self-proclaimed "accidental scientist" is a digital detective inventing new ways to tell if photos have been faked. Wisdom of the Crowds - Ask enough people to estimate something, and their combined guesses will get you surprisingly close to the right answer.