It’s also one of the most intensely studied topics in the world of astronomy and physics.
In April, Discover magazine published Corey S Powell’s “Out There” column with the headline “Dark matter is real. ‘Dark matter’ is a terrible name for it”, in which he says scientists “have been grappling with the mystery of dark matter for a long time, and I mean a looong time”.
He says the history of dark-matter investigations “goes back at least to 1906”, when physicist Henri Poincare speculated about the amount of “matière obscure” in the Milky Way. Or to 1846 and the discovery of the planet Neptune, whose existence had been inferred by its gravitational pull well before it was actually observed.
Our modern understanding of dark matter begins in the early 1930s with Swiss physicist Fritz Zwicky, called by Tom Ritchey, writing for the Swedish Morphological Society, “one of the broadest and most inventive scientists of his time”, and someone who “combined theoretical studies with eminently practical, humanitarian activities”.
Zwicky was born in Varna, Bulgaria, in 1898, the son of a Swiss merchant. At the age of six he was sent to Switzerland for schooling. In 1925 he moved to the US and went to work at the California Institute of Technology, in Pasadena.
Zwicky and German-born astronomer Walter Baade used a revolutionary new telescope at the Mount Palomar mountain-top observatory in southern California to photograph large areas of the sky quickly, with little distortion, to map out hundreds of thousands of galaxies, now called the Zwicky Catalogue of Galaxies.
They discovered that galaxies tended to cluster, “opening up a new chapter in the history of astronomy and cosmology”, Ritchey says.
At the same time, Zwicky applied the “virial theorem” of gravitational potential energy to the Coma cluster of galaxies, which led him to propose evidence of unseen mass, so starting off the debate on what is now called dark matter.
In his new book, Underland, Robert Macfarlane says Zwicky observed 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 properties is one of the grail-quests of modern physics.”
Zwicky and Baade also observed “bright novae” in order to determine the distance to galaxies, coining the term “supernova”, which Zwicky proposed marked the transition from ordinary stars to neutron stars – which he was the first to hypothesise – and were the origin of cosmic rays.
“This was an amazing (and correct) triple hypothesis and was an important step in the still on-going project to determine the size and age of the (visible) universe,” says Ritchey.
Related reading: Dark matter: when two massive things met