Node.js - searching for the "util" module; identify core modules

I'm really a big fan of Swagger's node module, but one thing is driving me crazy:

The sample app contains the following line (api/controller/hello_world.js):

var util = require('util');

But I just can't find this module. I tried to

• list it with npm list but nothing
• search for it with Spotlight (util.js)

My question is: How can I list the actually loaded modules in nodejs?