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Hot dark matter

Dark matter is matter that cannot be detected by electromagnetic radiation. Hence the dark. It is postulated to exist to explain how clusters and superclusters of galaxies formed after the big bang.

Data from galaxy rotation curves indicate that around 90% of the mass of a galaxy cannot be seen. It can only be detected by it's gravitational effect. There are several types of dark matter postulated to exist.

Hot dark matter consists of particles that travel with relativistic velocities. The best candidate for hot dark matter is the neutrino. Neutrinos have very small mass, do not partake in either the electromagnetic or the strong nuclear force and so are incredibly difficult to detect. This is why they are such good candidates for hot dark matter.

Hot dark matter cannot explain how individual galaxies formed from the big bang. The microwave background radiation as measured by the COBE satellite is very smooth and fast moving particles cannot clump together on this small scale from such as smooth initial clumping. To explain small scale structure in the Universe it is necessary to invoke cold dark matter. Hot dark matter therefore is nowadays always discussed as part of a mixed dark matter theory.

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