Although the Big Bang Theory comes into most people's minds when asked about how our universe may have come to be, there are actual several other theories floating around out there, some of which may blow your mind.
The Alien Simulation Theory
One theory circulating is that our lives and the reality we know is actually part of a simulation or game made by extraterrestrial life. Even creepier-human consciousness might be an illusion in itself and part of this simulation. Sounds like a great subject for a philosophy class
The Multiverse
Another theory which runs in the scientific community is that our universe is just a small part of a collection of other universes that could share similarities with our own. Therefore, the universe isn't really a "uni" verse but a multiverse. Which means that somewhere out there, there could be another version of yourself that pours the milk into the bowl before the cereal.
The Cyclic Universe Theory
And yet another theory states that the universe has already undergone, and will continue to go through several stages of expansion and contraction. Once our current universe expands to its fullest capacity, it will slowly start to collapse inward on itself until another "big bang" causes another expansion. This sounds somewhat painful, but luckily you and I will be long gone by the time it ever were to happen assuming this theory is true.
All of these sound like great plots for a science fiction novel, especially the alien simulation one. However it does make one wonder about the vastness of the universe, the enormity of it, and how much we, as humans, actually still don't know about it. We can have our theories as to how it came about and why, but we can never be 100% sure.