The universe is everything that exists: every star, every planet, every galaxy, every atom of every speck of dust, plus all the empty space in between. It began approximately 13.8 billion years ago in a sudden burst of growth and energy we call the Big Bang, and it has been expanding and cooling ever since. The further you look out into space, the further back in time you see, because the light has taken so long to reach us.
- Age of the universe13.8 billion yearsWorked out from the cosmic microwave background
- Observable universeapprox. 93 billion light yearsAcross (and growing every second)
- Galaxiesapprox. 2 trillionIn the observable universe alone
- Normal matterapprox. 5%Everything we can see, stars, planets, people
- Dark matterapprox. 27%Invisible, only known by its gravity
- Dark energyapprox. 68%Pushing the universe to expand faster
How did the universe begin?
The leading scientific idea is the Big Bang theory. About 13.8 billion years ago, the entire universe (every star, galaxy and atom that has ever existed) was packed into a single, tiny, mind-bogglingly hot point. Then in less than a billionth of a second, it expanded and cooled. The first atoms formed; gravity pulled clouds of gas together; and over hundreds of millions of years, those clouds collapsed into the first stars and galaxies.
What is the universe made of?
The strange thing is, the stars, planets and gas we can actually see only add up to about 5% of the universe. The other 95% is hidden. Roughly 27% is dark matter, an invisible kind of stuff that does not give off light but whose gravity pulls galaxies together. The remaining 68% is dark energy, a mysterious force that is making the universe expand faster and faster. Scientists are still working out what dark matter and dark energy actually are. It is one of the biggest open mysteries in physics.
Is the universe infinite?
Honest answer: we do not know. What we can see is the observable universe, the part close enough that light has had time to reach us in the 13.8 billion years since the Big Bang. That part is approximately 93 billion light years across (it is wider than its age in light years because space itself is stretching). Beyond the edge of the observable universe, more universe almost certainly carries on, but we cannot see it and we may never be able to.
Some scientists think the universe might be flat and infinite, stretching out forever in every direction. Others think it could curve back on itself like the surface of a giant sphere, so that if you travelled in a straight line for long enough you would end up back where you started. With current telescopes the universe looks flat to within tiny measurement errors, but the question is still open.
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