According to the standard cosmological model, the Universe started with a Big Bang about 14 billion years ago. During an early epoch of accelerated superluminal expansion, called inflation, a region of microscopic size stretched to a scale much larger than the visible Universe and our local geometry became flat. At the same time, quantum mechanical fluctuations of the vacuum generated primordial density fluctuations in the matter distribution. Gravity enhanced these inhomogeneities, seeding the formation of present-day structure.
The mass density of ordinary (baryonic) matter makes up only a fifth of the matter that led to the emergence of structure. The rest is in the form of an unknown dark matter component. Recently, the Universe entered a new phase of accelerated expansion due to the dominance of some dark vacuum energy density over the ever-lower matter density. This "dark energy" accounts for more than 70% of the mass-energy density of the Universe.