As Richard Carnes notes, the Big Bang Theory is most widely accepted theory of the origin of the universe and, at least to my knowledge, we have not yet observed evidence that contradicts this theory (if we did, we should have to discard it). This theory holds that our universe was created roughly 15 billion years ago at a singularity (physicist’s term for “we don’t know”) beginning with a cataclysm that created space and time, as well as all the matter and energy that constitute today’s universe.
Our early universe was extremely small, dense and hot, consisting of only energy, but it lasted only for a fraction of a second, expanding rapidly and cooling. During this stage the four fundamental forces (gravity, electromagnetism, and the strong and weak nuclear forces) became distinct (again in the terminology of physicists — if they existed before, then I suppose they were indistinct). Finally matter emerged from the process, enough to form the familiar (and not so familiar) bodies that populate our universe, including NFL linemen.
Now if that sounds a bit difficult to grasp, you are right — a “cataclysm that created space and time”! Theorists also argue that the laws of the universe as we understand them today did not apply prior to and during the initial part of The Big Bang (without space and time it is difficult to see how they could). And of course, unanswered (and perhaps never to be answered): What existed before the Big Bang (especially if space and time did not exist)? And what caused The Big Bang? Charlie Brown? No, he was smoking in the auditorium.
However there are other models that explain the evidence equally well. Some physicists argue that the Big Bang theory is based on a false premise — that the universe is built on an ever-expanding space-time, while others build theories based on a static universe, which is what was originally predicted by Einstein’s theory of general relativity, only later modified to accommodate the way the universe appears to be expanding. One interesting quote by internationally renowned astrophysicist/cosmologist/mathematician George F. R. Ellis (http://www.mth.uct.ac.za/~ellis/) may be of interest to the reader:
“People need to be aware that there is a range of models that could explain the observations ... For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations ... You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that.”
If all this seems to be stranger than a quark, you are right.
So if you want a truly mind-bending experience, take the cell phone out of your ear, quit playing games on iPads — study hard and become a physicist.