Lucky Fire, how the Big Bang theory (no pun intended) goes is like this:
At one point in the distant past (exact amount of time technically immeasurable), all of the matter in all the universe was compressed into an infinitesimal speck, which was also spinning at an incredibly fast speed. Then, from some sort of unknown stimulus, this speck exploded, releasing vast amounts of hydrogen and other trace elements, thus growing outward and expanding it into the universe as we know it now. The universe is probably still growing.
From a scientific standpoint I'm not exactly sure how that's possible, given the behavior of atoms and molecules. After a certain point, all the matter would start repelling each other, and even if all the universe was compressed into one area, it would have to be much much larger than just a speck. Furthermore, the theory doesn't really explain where the matter came from to begin with either. It also runs into problems with the law of conservation of angular momentum.*
(*In layman's terms, this law states that if an object is spinning in one direction, and something flies off that object, it's going to travel in the same direction, unless stopped by friction or something. This isn't a theory; it's proven scientific law. Taking that one step further, if this big bang speck was spinning in one direction, everything else springing off from it should also be spinning in the same direction. However, you can look at our solar system to take that theory into question - not all the planets orbit the sun in the same direction. Venus and iirc 2 other ones spin clockwise whereas the rest spin counterclockwise, and even the planets with satellites, like Jupiter and onward, have satellites spinning in opposite directions about them.)
A lot to take in all at once, I know. If you didn't get it all the first time, read it slower through a second time, or ask me for some clarification.