Clusters of galaxies
Background image: The galaxy cluster Abell 1689, seen both in optical and X-ray (purple colour).
Clusters of galaxies are the largest gravitationally bound objects, and constitute the nodes in the mesh of cosmic structure on which most of matter is located. They consist of hundreds to thousands of galaxies grouped together around a central dark-matter core inside a gas of baryonic particles. They emit radiation in optical wavelengths (stars), X-ray (bremsstrahlung from the baryonic gas), radio (non-thermal processes), and also induce a spectral distortion in the cosmic microwave background (the Sunyaev-Zel’dovich effect).
Whilst the first galaxies evolved some 800 million years after the Big Bang, clusters of galaxies were not formed until some 2 billion years later. The abundance and distribution of clusters of galaxies can be used to measure e.g. the amount of dark matter, dark energy and neutrinos in the Universe.