The oscillations came about in the following way: Regions of higher dark matter density cause increased gravitational attraction. But the fluctuations also had important short-term consequences: They excited oscillations in the cosmic plasma. These density fluctuations were the first seeds of the large-scale structure that we observe in the cosmos today, where matter is concentrated in galaxies and galaxy clusters. Quite soon after the big bang, the distribution of dark matter in the universe started to develop inhomogeneities, with the dark matter density slightly above average in some regions and slightly below in others. Consequently, the universe was completely opaque until it had cooled down sufficiently for electrons and nuclei to combine into stable atoms. On the other hand, a mixture of photons and electrically charged particles – such as electrons and nuclei – is subject to intense electromagnetic interactions. The only way that dark matter interacts with ordinary matter is via gravity. Also, there were photons and particles of what is called dark matter, as well as a number of other species of elementary particles that will not be important for what we are about to explore in this text. Instead, the cosmos was filled with elementary particles and, during all but the first minute of that era, with a plasma consisting of electrons and atomic nuclei. For the first 400,000 years after the big bang, the universe was so exceedingly hot that no stable atoms could exist.
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