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.
Since the Alpha Magnetic Spectrometer (AMS) was installed in May last year it has been gathering data from millions of light years beyond our galaxy, the Milky Way.
Until now, scientists have only been able to theorise the existence of dark matter, which makes up around 26% of the universe.
Its ghostly presence around galaxies exerts a gravitational effect that can be measured and observed as rings, but what it is made of remains to be proven.
The findings announced by Nasa reveal a positron spike "consistent" with the kind of interaction expected by one theory relating to dark matter.
But the scientists say the evidence is not yet "sufficiently conclusive" to rule out other explanations
Scientists may have found a hint of dark matter and could have started to unravel one of the universe's greatest mysteries.
Initial results from the Alpha Magnetic Spectrometer (AMS) have revealed tantalising evidence from fast moving particles.
They seem to bear the signature of collisions between atoms of dark matter, the mysterious "stuff" that cannot be seen or detected directly, yet which somehow binds the cosmos together.
However, the possibility that the particles have some other origin cannot yet be discounted.
Over the next few months, further analysis will show whether the "smoking gun" of dark matter really has been discovered.
The seven ton AMS is a super-sophisticated particle collector is attached to the outside of the International Space Station and is the most expensive experiments ever conducted in space, the AMS cost £1.32 billion.