The expanding universe predicted by general relativity has become firmly entrenched in modern science. As our ability to observe distant galaxies and map out the cosmos has improved, our picture of the universe has revealed some even more exotic features.
For a start, astronomers have been able to measure how fast distant spiral galaxies spin, and this shows that the outskirts of galaxies are rotating far too quickly to be reined in by the mass of the stars and gas at their centres. More matter is needed in galaxies to generate enough gravity to prevent galaxies from flying apart.
The popular explanation is that galaxies contain large quantities of other forms of matter 鈥� known as 鈥渄ark matter鈥� because it does not emit or reflect light. Dark matter is thought to clump around galaxies and clusters of galaxies in gigantic balls known as halos. Dark matter halos can be dense enough to significantly distort space-time and bend the path of any light rays that pass close by. This gravitational lensing has been observed in a number of clusters of galaxies, and is one of the strongest pieces of evidence for the existence of dark matter.
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But that鈥檚 not all. Cosmologists have been able to figure out how fast the universe expanded at different times in its history. This is done by measuring the distance to exploding stars called supernovae, and how quickly they are receding due to the expansion of space-time. The ground-breaking results from these observations, which emerged just over a decade ago, is that the expansion of the universe seems to be speeding up.
One explanation for this accelerating expansion is that the universe is permeated by an exotic form of energy, known as dark energy. Unlike ordinary matter and dark matter, which bend space-time in a way that draws masses together, dark energy pushes space apart, making it expand ever more quickly over time.
If we weigh up all the forms of matter and energy in the universe we end up with a striking conclusion: only 4 per cent of the universe is in the form of the matter we are familiar with. Around 24 per cent is dark matter and 72 per cent is dark energy.
This result emerged from the marriage of the general theory of relativity and modern astronomy and it has become a prime focus of physics. Experimenters and theorists are directing their efforts at trying to answer the burning questions: what exactly are dark matter and dark energy? And why do they have such strange properties?
Read more: Instant Expert: General relativity