Universe consists everything that exists physically, all forms of matter (and antimatter) and energy, entire space time, all physical laws that have been discovered or yet to be discovered and constans that govern these laws.
Estimated age of universe is about 14 billion years, found by astronomical observations. There are about 100 billion galaxies and each galaxy has stars ranging from ten million to 1 trillion. These stars orbit galaxy's center. A supermassive black hole is believed to have in center of most galaxies. Our own galaxy, Milky Way has a supermassive black hole in its center.
Astronomical observations indicate that universe is shaped by gravity, one of the four fundamental forces. Gravity is dominating at large cosmological distances where as other 3 fundamental forces(Strong force, Weak force and Electromagnetic force) dominate only at microscopic distances. General relativity is currently the most accurate theory of gravity, describing how gravity works.
Space time fabric is smooth and conntected and space has no curvature on large scales, as described by special relativity. But at quantum scale space time is highly tubulent, also called quantum foam. Universe appears to have 3 space dimensions and 1 time dimensions, but multiple dimensions exist at quantum scales.
The present overall density of the Universe is only about 9.9 x 10-30 grams/cm3. This mass-energy appears to consist of 73%dark energy, 23% dark matter and 4% ordinary matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume. The properties of dark energy and dark matter are largely unknown. Dark matter gravitates as ordinary matter, and thus works to slow the expansion of the Universe; and dark energy accelerates the expansion.
Universe is expanding as galaxies are receding from one another. This showed that in the past these galaxies were close to each other and if we move further back in time, we found that universe was a mere point. This gave rise to famous big bang theory of origin of universe.
The matter is spread uniformly (homogeneously) throughout the universe, when averaged over distances longer than 300 million light-years. However, on smaller length-scales, matter is observed to form "clumps", i.e., to cluster hierarchically; many atoms are condensed into stars, most stars into galaxies, most galaxies into clusters, superclusters and, finally, the largest-scale structures such as the Great Wall of galaxies. The observable matter of the Universe is also spread isotropically, meaning that no direction of observation seems different from any other; each region of the sky has roughly the same content. The Universe is also bathed in a highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.725 Kelvin. The hypothesis that the large-scale Universe is homogeneous and isotropic is known as the cosmological principle, which is supported by astronomical observations.
Astronomers and astrophysicists are trying to find a universal theory, which can explain everything in universe. But the biggest obstacle is uniting gravity and quantum mechanics. Universe seems to behave differently at quantum scales and cosmological scales. But String theory shows a ray of light in the way to find universal theory or 'Theory of Everything'.
