The main annual conference of the American Astronomical Society began this morning, and it didn't take long to roll into action. In one of the very first sessions, Glennys Farrar of New York University described some startling hints of a fifth force of nature, on top of the Fab Four: electromagnetism, gravity, and the two forces that govern atomic nuclei. The idea of a fifth force has a checkered history, and experiments seem to rule it out. But those experiments apply only to ordinary matter. They say nothing about dark matter.