The Large Hadron Collider (LHC), located at the Swiss-French border, is the largest and most complex machinery on earth. It smashes proton and heavy ion beams at nearly the speed of light, seeking answers to some of the most fundamental questions about the structure and interactions of our universe. The CMS detector, one of the main experiments at the LHC, is the world's heaviest microscope-plus-camera that observes and captures the movements of elementary particles created in LHC collisions. What are the main components of the CMS detector, and how do they function? How do physicists design a multi-purpose particle detector, as well as its upgrades? How can the CMS detector be used to probe the mysterious nature of dark matter? Using the CMS detector as the primary example, I will discuss the technology and physics purpose of particle detectors, and how the experiment may shed light on dark matter and our cosmological history.