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The Cosmic Microwave Background
Dark matter is collision-less, it just keeps going along its merry way. The prediction of the dark matter model is that the gravitational field should be where the galaxies are. And so here's what happens. A region of the early universe that is slightly over dense will start to contract. So in the early universe, there are deviations in density from place to place. Maybe they came from inflation, maybe they came from somewhere else, but somewhere early in the universe we don't know.