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In the early 1930s a Swiss astronomer called Fritz Zwicky was studying galaxy clusters through the telescopes of the California Institute of Technology when he noticed an anomaly of extraordinary implications. Clusters are groups of gravitationally bound galaxies, and Zwicky’s work involved measuring the speeds of revolution of individual galaxies in their orbits around the core of the cluster, in order to weigh the cluster as a whole. What Zwicky observed was that the galaxies were revolving much faster than expected, especially towards the outer reaches of the cluster. At such speeds, individual galaxies should have broken their gravitational hold on one another, dispersing the cluster.
There was, Zwicky determined, only one possible explanation. There had to be another source of gravity, powerful enough to hold the cluster together given the speeds of revolution of the observable bodies. But what could supply such huge gravitational field strength, sufficient to tether whole galaxies – and why could he not see this ‘missing mass’? Zwicky found no answers to his questions, but in asking them he began a hunt that continues today. His ‘missing mass’ is now known as ‘dark matter’ – and proving its existence and determining its qualities is one of the grail-quests of modern physics
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