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It is not only the theory of inflation that tells us we don't know all of the masses in the universe. In addition, there are observed facts that tell us the same. One of these is the chaotic motion of galaxies inside galaxy clusters. Galaxies don't appear singly, but rather inside such larger groupings. They are kept from escaping by the gravitational pull of the mass of the cluster. Individual galaxies move inside a cluster with velocities that are measurable by means of the Doppler effect. That's how we know these velocities to be so large that the gravity of the visible mass of the cluster does not suffices to hold the galaxies it contains together: Were there only the cluster's visible mass, its galaxies would have had to fly apart long ago. To keep all of them within the cluster, there must be about one hundred times more mass present than what is noticeable to us as visible matter.
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