The idea that the universe rapidly expanded in a fraction of a second right at the beginning is called inflation theory, it's a supplemental modification of the big bang theory and not all physicists that accept the big bang theory also accept inflation theory. There are also some theories that try to explain what there might have been 'before' the big bang, conformal cyclic cosmology for example. Then there's zero energy universe hypothesis, which suggests the universe may have arisen from a random quantum fluctuation - though a fluctuation of what is unclear.

So there are quite a few alternative variations on big bang theories. The observation that the universe is expanding seems solid, and the detection of the cosmic microwave background means something must have happened long ago that blasted out all that energy, seemingly everywhere at once. When you go beyond those though things start to get less certain.