Before 19th century, the idea of static universe filled with ordinary, visible matter, and radiation was widely accepted among the astronomers; even when the first relativistic model of universe was developed by Einstein in 1917, he added an extra term to the field equations in order to stabilize universe and keep it static; this term was called cosmological constant. Cosmological constant is the energy density of the vacuum that acts against gravitational attracting force. But when astronomical observations revealed the expansion of the universe, and astronomers hypothesized that our universe had begun by a Big Bang, Einstein came to this idea that there was not any need to the cosmological constant in his equations, because the expansion of the universe could be aftermath of the Big Bang, preventing gravitational attracting force to collapse our universe. The expansion of our universe was expected to be decelerating, but today's observations show that the expansion of our universe is accelerating; and in order to explain this accelerated expansion of the universe, cosmologists hypothesized the term 'dark energy' that is a fluid with negative pressure filled our universe, and it works against gravity and accelerates the expansion of today's universe. There are different models of dark energy; the first one is again cosmological constant, because it exerts repulsive force; and also there are dynamic and evolving models of dark energy, e.g. scalar field. On the other hand astronomers have found evidence that more than 85 percent of mass in our universe is invisible; they call it 'dark matter'. In addition to these two strange components, dark matter and dark energy, cosmologists have added a bizarre phenomenon to our universe in order to solve issues with Big Bang model of universe; this phenomenon is called inflation or exponential expansion of universe just after the Big Bang.
Big Bang model of the universe (with dark matter, dark energy, and inflation) is based on general theory of relativity and astronomical observations. The big issues with this model is the Big Bang itself, a singular point that gives birth to a planck scale universe, and inflation where general theory of relativity meets quantum mechanics.
