There are many theories as to what the eventual fate of the universe will be.
One leading contender is that the universe will continue to expand forever. This scenario is often called the “Big Freeze” because the universe will reach freezing temperatures when the supply of gas needed to form stars will eventually run out. Stars that remain will eventually die out, leaving nothing in the universe but black holes, which will eventually (over billions of billions of years) fade away too. This theory suggests that the universe will continue to expand to a point that it fades into nothing.
Another theory suggests that at some point, the universe will no longer expand but it will stop and begin contracting. The universe will get increasingly smaller and distant galaxies will approach and collide with one another. Temperatures will rise as matter collapses on itself, eventually shrinking back to a single point. Often called the “Big Crunch”, this theory claims that the universe has enough density and strength to hold back against the expansion that is currently happening. In other words, this theory is suggesting that the Big Bang will happen backwards, leaving some scientists to suggest that maybe another Big Bang will start the expansion process all over again. This is much like blowing up a balloon, letting the air out, and blowing it up again over and over, but taking trillions of years to do so. This cycle is often called the “Big Bounce”.
The Hubble eXtreme Deep Field (XDF) was completed in September 2012 and shows the farthest galaxies ever photographed. Dark energy is used to explain how galaxies are moving away from each other at a faster and faster rate.
The existence of dark matter and dark energy complicate these theories. Dark matter is used to explain why the universe holds as much mass, or “stuff” as it does. When astronomers first discovered that the largest galaxies had a much greater mass than the stars, dust and gas that they contained, they needed a way to explain this missing mass. Even though dark matter makes up such a significant portion of the universe, we can’t see it. This is because, to our knowledge, dark matter does not emit or reflect any form of radiation (including light or x-rays) so we can only recognize that it exists by how it interacts with what we can see.
Dark matter’s even more mysterious companion is dark energy. This term is given to the force responsible for making the universe expand at an increasing rate. Distant galaxies are moving away from us and this increasing speed of our universe’s growth is not well understood by scientists.
Current cosmology models suggest that “dark matter” and “dark energy” make up most of the universe (over 95%). This leaves only a mere 5% for ordinary matter like planets, stars and all the gas that exists between them. New theories have proposed that the density and mass of dark energy will increase with time. This means that the universe will not only continue to grow, but the speed at which it grows will accelerate faster and faster. Because of this, all non-dark matter will be ripped apart, starting with galaxies and eventually even individual stars and planets. This theory is called the “Big Rip” and it suggests that dark energy and the universe’s expansion will continue forever, tearing apart everything that exists, no matter how small.