In a scientific paper done in collaboration with James Hartle, Stephen Hawking was able to propose that the universe did not have to begin with a "Big Bang." The "Big Bang" concept has to do with what is known as a singularity. A singularity is a concept in physics wherein a dying star collapses in upon itself causing what is known as a Supernova. When a star dies and collapses in upon itself the gravitational pull can cause a catastrophic explosion that ejects most of its mass back into space. It is theorized that the whole universe has gone through this process causing what is known as the 'Big Bang!'
Well, in this paper Hawking shows that there is no need for a "Big Bang." Hawking proposes it in this way: "Jim Hartle and I showed how a universe like our own could be born without the troublesome singularity. It involved the use of what is called imaginary time. This may sound like science fiction but it is a well defined scientific concept that science fiction borrowed. The idea was that in imaginary time the universe has no boundary, no beginning or end, it just curls round on itself like the surface of the earth. I had to make it clear that the no boundary universe is just a proposal. But it has some interesting implications. Without boundaries the universe has no beginning and no end. We don't have to explain its creation. The universe simply exists. But the consequences of a no boundary proposal cannot be worked out fully without a complete quantum theory of gravity that will unite general relativity and quantum mechanics."