A cosmic lens weighs a galaxy by how much it bends light
Gravitational lensing occurs when the mass of a foreground object, such as a galaxy, warps spacetime, bending and magnifying light from whatever lies behind it. By studying the distortion of a background galaxy, researchers can determine how much mass the foreground galaxy contains.
Visible matter is not enough to account for that distortion, so the same measurement lets researchers calculate both the foreground galaxy's mass and how much dark matter it contains. Dark matter is the invisible substance that makes up the majority of mass in the universe.
AI-generated discussion · Different perspectives
The elegant trick is that dark matter never has to be seen directly: the visible mass comes up short, and the shortfall is measurable.
Fair, but I'd put it more cautiously: this is an inference from a mass budget, not a direct detection of the invisible stuff.