In inflationary cosmology, quantum fluctuations provide the energy for the expansion. According to quantum theory, the vacuum of space is far from empty. It seethes with particles and antiparticles constantly being created and destroyed. Energy from this vacuum can be tapped and is more than sufficient to trigger the era of explosive expansion dubbed the Big Bang. They are thought to correspond to slight variations in the distribution of matter at the moment when light and matter parted company, and radiation streamed freely into space.
Inflation also explains the overall uniformity of the universe.
Conventional Big Bang cosmology cannot account for how regions of the universe separated by distances so large that they have never even exchanged light signals can look so similar to each other. According to inflation theory, the universe began with regions so tiny that they were homogeneous. These regions then expanded into volumes vastly bigger than astronomers can ever observe. Inflation makes the cosmos not only uniform but also flat. Any curvature to space-time is stretched out by the expansion, like a cosmic version of a balloon stretched to enormous proportions.