Cosmic Inflation

When the Universe was very young, many scientists believe it went through a period of exponential expansion (note that expansion is exponential because that space seems to actually "grow" between two objects, rather than them moving apart). Evidence for this theory includes characteristics of the Universe today. A rapid expansion would flatten the Universe, which is reflected in the current Universe's structure.  The Universe is also roughly homogeneous, which means that a period of quick expansion may have delayed interactions between particles.  Also, based on the current information, the geometry of the Universe is the same in all directions which means its also isotropic, also a sign of rapid inflation.  Scientists have yet to explain this theory, but they have suggested the existence of an inflaton, a particle or field used to explain inflation.  But the real cause of this expansion, or proof that it even happened is currently out of reach.