Jocelyn Monroe, a postdoctoral fellow in physics at the Massachusetts Institute of Technology (M.I.T.), peeks through an opening in a neutron detector at the M.I.T. Laboratory for Nuclear Science. The detector, which Monroe built with her students, will help to quantify the background noise caused by neutrons in experiments searching for dark matter, the elusive stuff hypothesized to fill the universe with many times more mass than that contributed by ordinary matter.