What is dark matter made of

asks woolley:

A. It is matter in the universe that we can't see - cosmologists believe we can only see about 10% of the matter in the universe.

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Q. Can telescopes see dark matter
A. No. As well as visible radiation (i.e. everything we can see), telescopes have the ability to see some other types of radiation, but they can't detect it, either. Dark matter gives off no electromagnetic, infrared or ultraviolet radiation, no radio waves, and no x-rays or gamma rays.

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Q. So how do we know it's there
A. Scientists have worked out that it must be there because it has a gravitational pull on the matter we can see. The most noticeable gravitational effect is on the rotation of galaxies.

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Q. How does that work
A. Put simply, scientists can tell how fast and in what direction each star in the galaxy is orbiting about the centre of that galaxy. The orbital speed is determined by how the mass in that galaxy is distributed. If you were to make a graph showing the orbital speeds of the stars versus their distances from the centre of the galaxy, you'd see a curve - known as the 'rotation curve'.

If you looked at all the visible matter in a galaxy, and used the laws of gravitational physics to predict what the rotational curve would be, you would assume that the speed of the stars would decrease in a predictable fashion the further they were from the centre of the galaxy.

However, rotational curves level off and the stars which are further from the centre of the galaxy move faster than expected. The only thing that can explain this is dark matter.

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Q. So, what's dark matter made of
A. There are two theories:

• MACHOs (Massive Astrophysical Compact Halo Objects) - their existence has been confirmed experimentally in the Milky Way. No-one knows what they are exactly, of where they come from. Suggestions include primordial black holes and new types of astrophysical objects we have yet to learn about. An average MACHO is about half the mass of the sun, but there doesn't appear to be enough of them to account for all dark matter.
• WIMPs (Weakly Interacting Massive Particles) - these are enormous elementary particles that don't interact significantly with matter. (If they did, we would have found them). There are lots of them and together they could exert a strong gravitational pull.

To find out more, visit the Nasa universe site

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By Sheena Miller

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