What is It?
The Universe is the entire of what we would call 'space'. It includes earth, the sun, all of the stars you can see at night, plus many more you can't and all of the planets that float around these stars.
The universe is made up of galaxies. These are clumps of stars, planets, gas and dust that are held together by gravity. The galaxy that we are in is called the 'Milky Way'; most scientists believe it is held together by the gravity of a black hole (a very small star). Galaxy come in various shapes and sizes, our galaxy is a spiral shape.
To get an idea of the size of the universe. The milky way is 180 000 light years across, meaning it would take 180 000 years to travel from one side the the other at the speed of light. And there are billions of galaxies in the observable universe, possibly billions more outside of the area we can see.
Big Bang Theory
This is probably one of the most famous scientific discoveries, but it is an everyday phrase that people use without really knowing what it means.
The theory that is more or less the one we use today was proposed by someone who doesn't get alot of attention in the popular press. He was a Catholic Priest from Belgium called Georges Lemaître and put forward the theory in 1931.
The theory says that the universe began as a very small particle consisting of the entire universe squashed together. This particle then 'exploded', and when an explosion happens it pushes outwards (think of movies). This caused the universe to expand and it is believed that the universe continues to expand.
This expansion means that these galaxies mentioned above are moving away from each other. And this has been observed by astronomers; providing evidence for this theory.
Further evidence for the Big Bang is background radiation. Background radiation is the low levels of radiation that exist all around us; this radiation is picked up by an analouge television to produce 'snow' when it is not tuned. Background radiation in the universe is said to be left-over heat energy from the initial explosion of the universe.
Future of the Universe
If the universe is expanding, it means it has been possible for scientists to work out what might be the fate of the universe. However it depends upon the mass of the universe.
A common theory is that of the big crunch this will happen if the universe is above a critical mass. It says the universe will eventually stop expanding and then compact again. As it does this getting hotter. It will eventually compact into a single particle like at the beginning of the Big Bang Theory; many say that the universe will then be 'reborn' with another big bang and continue expanding and contracting forever.
Another possibility is if the mass is too low, in which case the universe will continue to expand forever, getting colder and colder as it does so; until it is eventually virtually empty and frozen.