Q&A: What is dark matter? And how do we know it exists?

What is dark matter?

It is everything we cannot see or detect in space. Scientists estimate that about 95 per cent of the Universe is composed of dark matter.

How do we know it exists if we cannot see it?

We know it is there from the gravitational effect it has on the objects we can see, like the stars and galaxies. Scientists estimate that about 24 per cent of the Universe is composed of this “missing mass”, with the rest made up of dark energy, which is forcing the Universe to expand at an ever-faster rate.

What is dark matter made of?

There have been two competing theories. One is that it is made of massive objects such as brown dwarf stars. The other that it is made of huge numbers of sub-atomic particles that do not interact with ordinary matter – it is these particles, known as neutralinos, that the Alpha Magnetic Spectrometer (AMS) is designed to detect.

How does the AMS work?

It cannot detect neutralinos directly, but can detect the particles given off when they collide in a process known as annihilation. The particles are in the form of electrons and their anti-matter partners, called positrons.

What are the results of AMS?

It has detected an excess of positrons, indicating the existence of neutralinos, but these could still come from distant pulsars – rotating neutron stars. More research is needed to resolve this.