"The existence of dark matter was first suggested in the early 1930's by the Swiss physicist Fritz Zwicky who calculated that the radial velocities of eight galaxies was 400 times greater than that expected by the shared gravity of luminous matter in those galaxies. The explanation given by Zwicky to his extraordinary find was to suggest the existence of what he called "dark matter", or matter which cannot be directly observed but can be inferred indirectly by its gravitational influence on visible matter. Analogously, imagine a caveman, who never saw a modern city, looking at New York at night. Naturally he will assume that New York is just a collection of light sources since all he can see is a variety of bright dots. Just like New York, space has much more then meets the eye."

Neat summary here.