Galaxies are immense collections of stars, dispersed in the vast space of the Universe. Majestic and timeless they float around, while nevertheless full of action in the form of exploding stars, black holes, and galactic collisions. Some are red and others are blue, some are flat while others are round, and some a big while others are small. Yet they are all governed by the same physical laws. What are galaxies made of, how are they created, and which processes can lead to this diversity? This talk will focus on the basic theories, as well as the latest results from the Cosmic Dawn Center, a research group under the Niels Bohr Institute and DTU Space, dedicated to unravel exactly these questions.