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Dark matter

Part of a map of dark matter in the UniversePart of a map of dark matter in the UniverseScientists think that a large part of the total mass of the Universe—about 85% of it—cannot be accounted for by the mass of objects we can see or detect. Only the small remaining fraction, 15%, is made up of atoms, the building blocks of galaxies, stars, planets and living things. Scientists call the mysterious "invisible" portion of the Universe dark matter. Dark matter cannot be detected using telescopes, because it does not give out or absorb light or any other form of electromagnetic radiation. This is why it is described as "dark." It is likely that dark matter is made up of types of subatomic particle that have yet to be discovered, called "Wimps".

Spinning galaxies

The effect of dark matter on galaxy rotation
Scientists know that galaxies appear to spin too quickly to hold themselves together. They should fly apart, yet they do not. Something else therefore is providing the gravitational force needed. Scientists say that it is invisible dark matter that is providing the mass that gives galaxies this gravitational force.

A galaxy cluster acting as a gravitational lensA galaxy cluster acting as a gravitational lens

Gravitational lensing

The first scientist to suspect the existence of dark matter was William Thomson, Lord Kelvin, in 1884. Estimating the mass of the galaxy to be greater than the mass of visible stars contained in it. Lord Kelvin concluded: "Many of our stars, perhaps a great majority of them, may be dark bodies."

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