Only God knows precisely what happened at the moment of creation, but physicists are beginning to get a clearer picture, as described by writer Bill Bryson:

In a single blinding pulse, a moment of glory much too swift and expansive for any form of words, the [universe] assumes heavenly dimensions, space beyond conception. In the first lively second (a second that many cosmologists will devote careers to shaving into ever-finer wafers) is produced gravity and the other forces that govern physics. In less than a minute the universe is a million billion miles across and growing fast. There is a lot of heat now, ten billion degrees of it, enough to begin the nuclear reactions that create the lighter elements, —principally hydrogen and helium, with a dash (about one atom in a hundred million) of lithium. In three minutes, 98 percent of all the matter there is or will ever be has been produced. We have a universe. It is a place of the most wondrous and gratifying possibility, and beautiful, too. And it was done in about the time it takes to make a sandwich.