PIR =Pyroelectric IR (infrared). Pyroelectrics make voltage when heated or cooled. IR light is really good at changing something's temperature. The white dome is actually a lens that helps focus light coming from all directions in front of it to an IR pyroelectric crystal in the center. The circuit board detects if incoming IR light is rapidly changing at the same wavelengths that people emit by just being warm.
Yes, you emit light, even in the dark! This sensor detects changes in that light. Here is a picture from an IR camera.
IR light goes through plastics