Astronomers today understand that the universe is full of a mysterious substance they call “dark matter” (because it doesn’t give off any light or other radiation we can detect.)  Dr. Tom Shutt (of the Stanford Linear Accelerator Center) discusses the motivation behind the multi-decade, world-wide effort to test the idea that dark matter is in the form of particles as small as a neutrino but as heavy as an atom.  He describes the experiment he is involved with, that uses 7 tons of liquefied Xenon to measure how these particles interact with normal matter.