We think we now know quite accurately what the universe is like. It consists of a vast expanse of space, in which matter is dotted about in the form of galaxies.
Each galaxy, spiralling round like a gigantic Catherine wheel, is made up of billions of stars, which are like our Sun. And many of these stars almost certainly have planets circling around them, much as our Sun has. And the likelihood is that some of these planets are inhabited by different kinds of life forms.
Just how old is this universe of ours? Well, by studying the rocks, we know how old the Earth is - about 4,600 million years. So this is also presumably the age of the Sun and the rest of the solar system.
But suns are being born all the time in space, so the universe would be expected to be more than 4,600 million years old.
To find out how old, astronomers look at distant galaxies. They find that all the galaxies are rushing away; away from us and away from each other. And the farther they are away, the faster they are moving. It appears that the whole of space is getting bigger. This idea is known as the expanding universe.
If the universe is expanding, this means that in the past it was smaller and its matter was packed more tightly together. And in theory if we go back in time far enough, we should reach a time when all the matter in the universe was concentrated in one spot.
And, broadly speaking, that is what astronomers think happened. They think it happened about 15,000 million years ago.
They reckon that 15,000 million years ago all the matter, and energy, in the universe was created, and the universe started to expand, in a gigantic explosion they call the Big Bang.
The Big Bang didn't just create matter and energy. It created space as well. It also created time. There was no matter, energy, space or time until the Big Bang. We can't talk about 'before the Big Bang', because time didn't exist until the Big Bang.
At the instant of the Big Bang, the universe was infinitely hot and full only of energy. But in seconds the universe had expanded and cooled down enough for the energy to be transformed into atomic particles, such as protons, neutrons and electrons.
After a few minutes, as the temperature fell further, the particles began to join together to form atomic nuclei, notably nuclei of helium.
It took several hundred thousand years before the atomic nuclei and the electrons were able to join together to form atoms. The protons combined with electrons to form hydrogen, the helium nuclei to form helium. Hydrogen and helium are still the most plentiful elements in the universe even today. Hydrogen is the main 'fuel' stars 'burn' to produce the energy to keep shining.
The universe continued to expand and cool down. Much later, after perhaps 2,000 million years, clouds of hydrogen and helium gases began to collapse under gravity and form the first stars and galaxies. Star and galaxy formation has been going on ever since, creating the universe as it is today.
But what of the future? No one, of course, can predict what the future holds, but there seem two possible options. It depends on whether we live in an open or closed universe.
If the universe is open, the galaxies will continue to rush headlong through space, resulting in a universe of ever- increasing size. This will continue until all the stars and galaxies die and fade away billions upon billions of years hence.
However, if there is enough matter in the universe, the gravity associated with this matter will in time start to slow down the galaxies and the expansion of the universe. This is the idea of the closed universe.
There does not appear to be enough visible matter in the universe to make it closed. But it is possible that there is a lot of matter we can't see, called dark matter. If so, then the universe might well be closed.
A closed universe will one day stop expanding, and indeed start to contract. All the galaxies will start moving closer together, the reverse of what is observed to be happening at present.
The closed universe will continue to shrink, smaller and smaller until all its matter will be squeezed into a single point. The universe will then disappear in an event that astronomers call the Big Crunch. The Big Crunch would be the opposite of the Big Bang, and signal the end of the universe.
Or would it? The Big Crunch could well trigger off another Big Bang, which would create a new universe. This new universe would expand, creating stars and galaxies like the ones we see today.
Then that universe might end in another Big Crunch, to be followed by still another Big Bang. This idea, of a continuous cycle of expansion and contraction, of Big Bang and Big Crunch, is known as the oscillating universe.
