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Cosmologists are able to measure the contents of our universe in a number of ways. Whether it is by counting how many galaxies there are in the sky and measuring how they group together, or by measuring the faint radiation left over from the very early universe, the emerging picture is very similar. The results can be nicely summed up with a Cosmic Pie.
We are most familiar with just 5% of the matter and energy in the Universe, the stuff that we are made of, atoms or star stuff. The rest is dark and bizarre, but we have started to explore the unknown. Dark matter, which makes up about 26% of the Cosmic Pie does not shine like stars do and it hardly interacts with itself or other particles. We know it must exist because it has gravitational influence, it exerts a gravitational force on other matter. The remaining 69%, is even more strange.
Although called dark energy, as though the universe had an unknown form of ‘energy’, it need not be an energy at all. Dark energy is used to explain the fact that the universe is not just expanding at a constant rate, but is actually accelerating, with galaxies racing away from us in all directions. It is as though there was an ‘unseen’ force or energy driving this cosmic acceleration, with dark energy counter-acting gravity. While dark energy could be a fundamental property of space-time itself, it could also signal that our current understanding of gravity, while very good, is incomplete. That is to say, that nature is weirder than we thought, and we might need to revise our understanding of gravity on cosmic scales.
For more information on Planck and the Cosmic Microwave Background see the Planck webpages.