Scientists now believe that ordinary matter - the kind that makes up stars, planets and ourselves - accounts for just 4% of the entire universe.
Dark matter is thought to make up around 26% of the universe.
The remaining 70% of the cosmos is thought to consist of dark energy, an even bigger enigma than dark matter, which appears to be driving galaxies apart at an accelerating rate.
A leading theory suggests that dark matter is composed of exotic particles known as Wimps (weakly interacting massive particles).
If Wimps exist, they would annihilate each other when they collide to release electrons and their antimatter equivalent, positrons.
It is the positrons left behind by dark matter collisions that AMS is looking for. By analysing the ratio of positrons to electrons and measuring the energy of the particles, scientists hope to find the first solid clues to the nature of dark matter.
Another telling sign is the direction the positrons are coming from. If they are generated by dark matter, they should be spread evenly through space. But if they are created by a normal process, such as an exploding star, they would originate from a single direction.