Cosmology is the part of astronomy that deals with the Universe on its largest scales: its origin, its structure, and its evolution.
That is, cosmology doesn't care about trifles such as planets and comets, and doesn't even care much for stars and galaxies on their own, but is more concerned with how the galaxies and, in particular, the dark matter halos hosting them, are distributed in the Universe, how much, and what kind of, matter exists in the Universe, how and when in its history the Universe underwent "global" changes, etc.
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