Dark matter; experiments with memory loss in mice; “digital detective” Hany Farid, professor of computer science at Dartmouth; and the “wisdom of the crowd.”

"Dark Matter"--Host Neil deGrasse Tyson reports from a half-mile underground in an abandoned mine, where scientists are using special detectors to look for evidence of a ghostly substance that they believe makes up most of the matter of the universe - a hypothetical entity called dark matter.

"Alzheimer's/Memory Mice"--Researchers have found that mice with induced memory loss are able to retrieve memories either by being put into an enriched environment or by being given a drug that promotes beneficial gene activity in their brains. Such new insights may lead to a better understanding of dementia and other memory-impairment disorders, such as Alzheimer's, in humans.

"Hany Farid"--Is seeing believing? In this age of easy photo manipulation, sometimes only a digital detective can tell. Enter Hany Farid, professor of computer science at Dartmouth College. He developed a program that can unmask photo tampering by analyzing light sources and other subtle features in an image - a service that turns out to be in high demand, especially during political campaigns.

"Wisdom of the Crowd"--Famed British scientist Sir Francis Galton (1822 to 1911), a proud aristocrat, thought he was proving the ignorance of the masses in his observation about a country-fair competition in which entrants were asked to guess the exact weight of an ox. The correct answer was 1,198 pounds; understandably, none of the 800 contestants got it exactly. "See!" said Sir Francis. "Decisions should be left to higher classes" - or sentiments to that effect. However, Sir Francis failed to realize that graphing all the guesses and determining their median produces the right answer and shows the "wisdom of the crowd."