We live on a planet that is 13,000 km in diameter, orbits around a star that is 1.4 million km in diameter, and resides in a galaxy that measures 100,000 light years in diameter but only 1000 light years in height. The closest galaxy to us is the Canis Major dwarf galaxy, which was discovered in 2003 and is located about 25,000 light years from our solar system and 42,000 light years from the centre of our galaxy.
In the 1930s Harlow Shapley discovered that individual galaxies are grouped into clusters. In the following 30 years, thousands more galaxy clusters were discovered by George Abell and Fritz Zwicky. Abell suggested that clusters of galaxies are “fundamental condensations of matter in space” that can be used to investigate the formation and evolution of the universe. Clusters can contain as many as 10,000 galaxies, or as little as 30 in our “local group”.
Clusters of galaxies are part of the visible universe of stars, gas and dust. However, this visible (or baryonic) part of the universe makes up only 5% of the universe’s total mass–energy budget. The rest resides in dark matter (23%) and dark energy (72%) – intangible forms of matter and energy that have not yet been directly observed.
The discovery of clusters of galaxies in the 1930s prompted Zwicky to introduced the idea of dark matter to account for the missing baryonic mass needed to...
