But she found that this is not the case. Large amounts of mass would account for the unexpected speed, but we don't see the mass that should be there. To aid your understanding of this, think of how planets revolve around the Sun in our solar system . If there were a solid sphere of mass between the Sun and Pluto, than Pluto's orbital period would be the same as Mercury's. Because we can't see it, the mass is called dark matter .
Scientists tend to categorize everything and matter is no different.
The matter you are familiar with, matter composed of neutrons and protons, is called baryonic matter. Non-baryonic matter also exists, but is generally difficult to detect. Professor Sadoulet's experiment is looking for exotic, non-baryonic particles called WIMPs . WIMP stands for Weakly Interacting Massive Particle. There is a great deal of theoretical work which suggests that WIMPs exist and probably account for a large fraction of dark matter.