In the standard picture, most of the Universe is composed of dark matter, whose nature is unknown. Protons, electrons and neutrons, which compose gas and stars, make up the rest. They interact with dark matter purely through gravity, which determines the evolution of the Universe on large scales. The Universe emerged from the Big Bang with small inhomogeneities. These eventually grew into lumps, called haloes, by attracting surrounding matter gravitationally. The competition between radiative colling and gravitational heating determines the fate of gas in these haloes. In low-mass haloes, cooling dominates. Galaxies grow through the accretion of gas that falls to the centre in cold flows.
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1 In the beginning God created the heaven and the earth.
2 And the earth was without form, and void; and darkness was upon the
face of the deep. And the Spirit of God moved upon the face of the
waters.
3 And God said, Let there be light: and there was light.
4 And God saw the light, that it was good: and God divided the light from the darkness.
5 And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
6 And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
7 And God made the firmament, and divided the waters which were under
the firmament from the waters which were above the firmament: and it
was so.
8 And God called the firmament Heaven. And the evening and the morning were the second day.
9 And God said, Let the waters under the heaven be gathered together
unto one place, and let the dry land appear: and it was so.
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