More than 2 kilometers below the surface of northern Ontario, suspended in 345,000 liters of ultra-pure water, there’s a perfect sphere. Scientists continuously monitor this chamber, looking for a glimmer of light in the darkness. Why? Because that light would indicate the presence of one of the universe’s greatest mysteries: dark matter. Jenna Saffin describes how this dark matter detector works. Lesson by Jenna Saffin, animation by TED-Ed.