Observing Dark Matter in the Wild

Most of the matter in the universe is in a form that we can see only indirectly, through its gravitational effects. This “dark matter” has so far eluded all attempts to detect and characterize it more directly, so we know more about what dark matter is not than about what it is.  I will first review the gravitational evidence; describe why the dark matter hypothesis has outcompeted alternative hypotheses; and summarize what we have been able to deduce about dark matter’s properties. Then I will focus more on what we can learn about dark matter by observing it “in the wild”–that is, by observing its interactions with galaxies and intergalactic gas rather than by trying to capture it in a detector.