"The Dark Matters"
An anonymous question appeared on my board recently: if most of the matter in the universe is "dark", why can't we detect it via other means, like gravitational lensing?
Indeed, we do! From our understanding of general relativity, the presence of matter and energy warp and flex spacetime like a (four dimensional) rubber sheet. And beams of light like to travel in straight lines, but the spacetime "underneath" them is warped, forcing the light to follow curving paths. This means that a massive object can bend light around it like a lens...hence, gravitational lensing.
The measurement of this effect around the sun was one of the first pieces of evidence that Einstein got it right, and now we use it routinely to study matter all around the universe, including the dark kind.
"Dark" matter is really invisible matter - it turns out that most of the matter in the universe simply doesn't interact with light. We're not exactly sure what it is yet, but tools like gravitational lensing are helping us to study it. Even when we can't see it directly, it bends the path of light around it, so we know it's there and we can learn more about it.