Formation of Galaxies and Stars The Universe looks very homogeneous and isotropic on large scales, i.e., very uniform and similar in all directions. For example, the cosmic background radiation has very small ripples which are less than a tenth of a thousandth (10-4) of its mean intensity. All the structures in the Universe, including stars, galaxies and clusters of these, are believed to have originated from these cosmic ripples. Gravitational instabilities caused the contrasts between the denser and less dense regions to grow. Collisions between pieces of matter in the denser regions caused them to collapse and heat up, leading to the formation of stars. Galaxies are thought to have been formed by the amalgamations of smaller collections of stars. Many galaxies are known to have very dense regions in their cores, which are thought to be massive black holes. Simulations of structure formation in various models are made available by the Virgo Collaboration. Observations of very early structures have recently been made at ESO. Open questions One of the sets of major open questions in this picture is where the matter came from, why we see no large amounts of antimatter in the Universe, and how to explain the proportions of matter and radiation in the Universe. Another problem is to explain how quickly the galaxies formed after the Big Bang. It is thought that the gravitational instabilities leading to structure formation were accelerated by the attractions between unseen particles of dark matter. There is observational evidence from astrophysics and cosmology that most of the matter in the Universe is invisible, and experiments are now being made using particle accelerators and other methods to search for these dark matter particles.

An image of RCW38 obtained with the Very Large Telescope (VLT). This is a region in the Milky Way at a distance of about 5,000 light years, where stars which have recently formed in clouds of gas and dust are still heavily obscured and cannot be observed in the visible part of the spectrum.