In 1933 Fritz Zwicky (1898-1974) discovered that the galaxies in the Coma Cluster were moving too rapidly relative to one another for them to be bound together by their mutual gravitational attraction alone. This suggested that there had to be much more mass than was visible, keeping the galaxies from flying out of the cluster. This was an early indication of missing mass or dark matter.
In the early 1990s it was thought that gravity would eventually slow the expansion of the Universe. In 1998 the Hubble Space Telescope observations of distant galaxies showed that a long while ago the Universe was actually expanding more slowly than it is today. The expansion of the Universe has been accelerating but what was causing it? The explanation is not yet known but it has been given a name – dark energy.
Little is known about dark energy but its amount has been calculated from the expansion of the Universe. About seventy percent of the Universe is dark energy, about twenty five percent is dark matter, and ‘normal’ matter, i.e. everything that can be seen, adds up to less than five percent.
Einstein realised that space is not ’empty’. He predicted that more space could come into existence and that it possesses its own energy. As more space comes into existence, more of this energy would appear and cause the Universe to expand faster and faster. However, the calculation of this vacuum energy gave an answer that was far too big.
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