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Everything that exists – stars, planets, galaxies, and all that lies between – makes up the Universe. Scientists believe the Universe is 4 per cent ordinary matter, 23 per cent dark matter, and 73 per cent dark energy. Almost nothing is known about dark energy, but this is the name given to something that appears to exert a force making the Universe expand. Forces, such as gravity and the laws of physics and chemistry, determine what matter is like and how it behaves.
Looking up at the heavens, we see matter in the form of stars, planets, and in the glowing clouds of gas and dust known as nebulas. These are visible forms of matter. However, astronomers believe that over 90 percent of the Universe is made up of invisible matter, known as dark matter.
The Earth we live on seems big and very important to us. But in the Universe as a whole, it is a tiny, and very insignificant, speck of rock. To put things into perspective, Earth is just a small planet in the Solar System, part of a family of bodies that circle round the Sun. The Sun is just one of billions of stars in a great star island that makes up our Galaxy. And this Galaxy is just one of billions that make up a Universe bigger than most of us can imagine.
In the last 100 years cities have grown to house nearly half of the 6 billion humans that live on Earth. Thelargest cities cover areas tens of kilometres wide.
Earth looks beautiful from space. It appears mainly blue because of its vast oceans – water covers nearly two-thirds of our planet. Wisps of white clouds fleck the atmosphere. The continents appear brown and green. Earth is 12,756 km (7,926 miles) across at the equator.
Nine planets, and other smaller bodies, circle the Sun to make up the Solar System. Distances across the Universe are so vast that they are measured in light years – the distance light travels in one year, or 10 million million km (6 million million miles). The Solar System measures about 1 light year across.
The Sun is one of around 200 billion stars in the Galaxy, our local star island in space. It sits in one of the Galaxy’s spiral arms, about 25,000 light years from the centre. The Galaxy measures about 100,000 lightyears across but is only around 2,000 light years thick.
Our Galaxy is one of tens of billions ofgalaxies in the Universe. Galaxies are found in groups, or clusters, which in turn gather together to form superclusters. These interconnecting superclusters and the spaces, or voids, between them make up the Universe. Astronomers believe that almost all the galaxies are rushing away from us – and each other – at high speed, and they move faster thefurther apart they are. This tells us that the Universe is expanding. Astronomers believe that an explosion, known as the Big Bang, started off this expansion 13.7 billionyears ago.
The Earth is wrapped inside a layer ofair we call the atmosphere. At a height of about 300 km (200 miles) above Earth, few traces of air remain.Astronauts in orbiting spacecraft have spectacular views of this region and are able to see the blue of the Earth’s atmosphere gradually merge into the inky blackness of empty space.
The moons Io and Europa are seen here travelling across the face of Jupiter. They are kept in orbit by Jupiter’s powerful gravitational pull. Gravity is a dominant force in the Universe, and holds together systemssuch as galaxies. It is one of the four fundamental forces, and is the only one that can act over vast distances. Electromagnetism is the force that acts between all substances with an electric charge. The strong force and the weak force occur only in the nuclei of atoms.

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