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When we look out at the universe – even with the most powerful of telescopes – we can only see a fraction of the matter we know must be there. In fact, for every gram's worth of atoms in the universe, there is at least five times more invisible material called "dark matter". So far scientists have failed to detect it, despite spending decades searching.

The reason we know it exists is because of the gravitational pull of galaxy clusters and other phenomena we observe. The matter we can see in a cluster isn't enough to hold it together by gravity alone, meaning some additional invisible or obscure matter must be present. But we have no idea what it is – it could be made up of new, yet undiscovered particles.

There are four fundamental forces that a could interact with. There is the strong force that binds together the ; the weak force which governs the decay of particles such as radioactivity; an electromagnetic force that mediates the force between charged particles; and the gravitational force which governs gravitational interaction. To observe matter in space we need it to interact via the electromagnetic force, as this involves the release of light or other electromagnetic radiation that a telescope can register.

There are quite a few candidates already – each with its own particular way of interacting. However, some theories are more likely to be successful than others. Here are the five candidates for particles that I think have the best chance.

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