Ah ha! This sounds like a problem for a Master of Physics. (That?s me btw
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Dark matter is inferred to exist from the velocity of rotation curves of galaxies, measure from the light emitted. If dark matter didn?t exist/wasn?t present then the velocity of rotation would start to fall at high radii from the galactic cores, but this is not observed, and therefore there must be some form of matter surrounding the galaxies to account for this. This is dark matter.
Although dark matter is known to exist, and a few things are known about it (i.e. it?s temperature), it hasn't yet been directly observed, hence why its called dark matter.