Astronomers believe that the Big Bang took place 13.7 billion years ago, and that galaxies began to form 1–2 billion years later. Our Solar System was not created until about 4.6 billion years ago, with primitive single-celled life appearing on Earth about 1 billion years later. It was not until around 600 million years ago that an explosion of life occurred, in the Cambrian Period of Earth’s history. The first dinosaurs evolved 230 million years ago, and man’s earliest ancestors just 4 million years ago.

The Big Bang created an incredibly hot Universe a fraction of the size of an atom. It immediately started to cool and expand, for a brief moment growing at a tremendous rate, in a process called inflation. In less than a millisecond, the first matter was created but, for thousands of years, the Universe was dominated by radiation.

The first forms of matter created were the tiniest and most basic particles of matter, such as quarks. Scientists today try to recreate what happened by smashing particles in a particle accelerator and studying the tracks. As the Universe cooled, these particles combined to form protons and neutrons, which later joined to form the nuclei of atoms.

After 300,000 years, nuclei began to capture electrons and form the first atoms. This cosmic microwave map reveals what the Universe was like after 380,000 years. The red and yellow areas are slightly warmer than the blue and green ones and are a sign that matter was clumping.

As matter was drawn together by gravity, the first stars and galaxies were born. This Hubble Space Telescope picture shows galaxies 2.2 billion light years away, and many more remote galaxies beyond. The gravity of the cluster, including its invisible DARK MATTER, act like a lens to magnify the images of the more distant galaxies.

The composition of the Universe continues to change. These two galaxies are colliding, and flinging out streams of stars. The Universe is also still expanding. Because of an effect called REDSHIFT, astronomers know that almost all galaxies are accelerating away from each other. It is not the galaxies themselves that expand – they are held together by gravity – but the vast distances between the galaxies.

By analysing the spectrum of light from a star or galaxy, astronomers can tell how fast it is moving, and whether it is moving towards or away from Earth. If an object is moving away from Earth, its light shifts to longer, redder wavelengths, an effect known as redshift. We know the Universe is expanding as almost all galaxies show redshift, and are rushing apart.

The Universe is made up of matter and energy. Stars and galaxies are forms of matter that we can see. But there are also forms of matter that we cannot see, called dark matter. We know that dark matter exists because of the effects of its gravity. Astronomers believe that dark matter might account for up to 90 per cent of the matter in the Universe.