Creationism
The idea that God was so bored out of his mind he spent 6 days creating everything in the Universe, and for good measure put in several jokes to fool us into believing it must have taken him much longer. It is clear from this that God was not a graduate student, for if he was he'd have rested for six days and pulled an all-nighter sunday. One theory is that this is the case, thereby explaining dark matter as being composed of discarded Mt. Dew bottles.
According to this theory, He created everything in media res, ie, right in the middle of the action. Although the Universe is only 6000 years old, we see light from stars which appears to have taken billions of years to reach Earth. Although radioactive minerals in rocks appears to have taken billions of years to decay to the state we see in nature, the rocks are really only 6000 years old.
This is either because God is really really sneaky, or has a sadistically vicious sense of humor, or because Creationists choose to interpret the Bible in such a way so it seems that way. Any marginally intelligent person would not take the bible on a literal basis, but... well, we are talking about Creationists here.
But basically, an omnipotent being can make a Universe any way He wants. He can make it so that everyone chooses of their own free will to be good; and evil and suffering don't exist. He can teach each and every human soul to reach salvation, no matter how long it takes (He's got all the time in the Universe and more if He wants it.)
According to Creationism, that's not what God did. Instead, He made malaria, elephantiasis, bone-break fever and churches which ceilings fall during religious service. The theory of Creationism implies that a perfect, benevolent, all-powerful Being created a Universe which He knew would produce not only Jeffery Dahmer but The Backstreet Boys as well. - Have you ever heard the song "I never promised you a Rose Garden"? Well, the singer is not God, who describes Himself as perfect and good despite the obvious evidence to the contrary.
Needless to say, Creationism is highly regarded because it is almost perfectly illogical and incomprehensible -- harder to make sense of than Einstein's general relativity, quantum mechanics, and Keynesian economics combined. Something this nutty just has to be true. One of the greatest open questions in theoretical Creationism is whether the stupidity of the theory is uncountably infinite. One promising line of research focuses on creating a one-to-one correspondence between contradictions in the Bible and stupid remarks by Beavis and Butthead, who are known to be countably stupid.
On the other hand, some hold that Creationism has fallen into disrepute and has been called by renowned scientists across the globe: "A disease of the mind, in the same family as coprophilia, necrophilia, Gonorrhea Lectim, and fret wanking". ~ Oscar Wilde
Or conversely,
The main article sets up several straw-man arguments and is banking on the ignorance of the readers. To find out what Creationists really believe I suggest that the curious reader visit http://www.answersingenesis.org, and then die laughing. I mean, honestly! The Discovery Institute is also good for a laugh. These people are coming to a school board near you. Quake with fear and tremble before your (putative) maker.
And of course, you can put two creationists in a room and you can come out with any number of distinct theories, including the quantities 3, 1/2, i, 1/3, 3 2/4, and .
Alternate Theories
An alternate theory is that the universe is really just a giant Pez dispenser for God.
It may also be possible that the Universe is a cosmic soap opera for a Supremely bored God.
Another popular belief is that the Flying Spaghetti Monster created everything. Given the noodly texture of reality, they might be on to something.
See also: Intelligent Design, Carl Peterson, Sphincter Wanking.