The universe has not always appeared as it does today. Most theories agree that the universe of 15 billion years ago was vastly different that the present universe. Three theories have gained widespread acceptance in the scientific community:

The Big Bang Theory: This theory states that 15 billion years ago the universe and all of the matter in it was compressed into a very small, dense and hot fireball. At this time there was no such thing as planets, stars or galaxies. The entire universe consisted of nothing more than the rubble of smashed and broken atoms. Everything, including space itself, was squeezed into this fireball by the tremendous forces of gravity. We really don't know how long the universe existed in this condition. We do know that about 15 billion years ago, something big happened. To call it just "an explosion" just doesn't do justice to this event. This universe of ours started to expand. Sort of like a balloon being inflated, the universe began to get larger. As the fireball expanded, it began to cool off. This process took millions and millions of years. Eventually, the matter within the universe cooled enough for simple hydrogen atoms to form. Lumps and clumps of this expanding matter began to form. These early lumps and clumps of material would eventually become the first stars and galaxies. Concentrations within these lumps and clumps eventually became planets and moons. 15 billion years later, after a lot of expanding and cooling, the universe appears as it does today. It is roughly spherical in shape, perhaps 30 billion light years from side to side. In this universe are about 100-200 billion galaxies, each containing about 100-200 billion stars. According to the Big Bang Theory, nothing will ever be as powerful as the force of the big bang. Even gravity will never be able to reverse the expansion that began so long ago. In the very distant future, when all the stars have used us their fuel and the last galaxies' light winks out, the universe will be cold, dark and empty, and still expanding ever larger. The expansion will continue forever. Not exactly a bright and cheery future, but keep in mind that "distant future" means hundreds of billions of years from now. Our universe has only been around for a mere 15 billion years, so there is still a lot of time left.

The Oscillating Universe Theory: This is a slightly more modern version of the Big Bang Theory. It has found acceptance by about half of the scientific community today. The other half tends to accept the Big Bang Theory. The first part of the theory is exactly the same as what you learned for the Big Bang Theory.  Fifteen billion years ago the universe formed with a big bang, it began to expand, and galaxies, stars and ultimately YOU formed from lumps and clumps of the expanding universe.  The basic difference is in the power of gravity, and therefore, how much matter there actually is in our universe. Recall that in the big bang theory, the most powerful force ever is thought to be the force of the big bang. Nothing, not even gravity, will have enough power to stop the expansion of the universe. This theory contends that the ultimate winner in the battle of the forces will be the force of gravity. Proponents of this theory believe that there IS enough matter in the universe to make enough gravity, to one day stop the expansion. According to this theory, gravity will eventually slow the expansion of the universe, gradually brining it to a full stop. After a time, the ceaseless force of gravity will actually reverse the expansion, and cause the universe to begin to contract. This is a time period known as the "Big Crunch".  This contraction will go on for many years, until all matter and space in the universe is once more contained in a fireball. This fireball for exist for some amount of time, then it will to experience another big bang! (It's only reasonable to expect that if it happened once, it's likely to happen again!!) After this second "bang", this universe will begin an era of expansion once again, only to see the expansion stopped by gravity, begin contracting and end up as a fireball again. According to this theory, this cycle will continue forever, as the universe forms, destroys and reforms itself again and again!

The Steady State Theory: This theory is much different from the other two. The steady state theory really isn't all that well accepted by the scientific community these days, but it used to be, so I'm including it here for historical purposes. The steady state theory has four points:

• The universe never "began" and it will never "end", it simply has always existed and always will!
• The universe has always appeared and will always appear much as it does now.
• As some galaxies die, new ones will take their place.
• Stars produce the raw materials from which other stars are constructed