It is worth emphasizing that those distant regions of space are taken to exist and be part of reality as much as we are; yet we can never interact with them, even in principle. The spatial region within which we can affect and be affected is denoted as the "observable universe".
Strictly speaking, the observable Universe depends on the location of the observer. By travelling, an observer can come into contact with a greater region of space-time than an observer who remains still, so that the observable Universe for the former is larger than for the latter. Nevertheless, even the most rapid traveler may not be able to interact with all of space. Typically, the 'observable Universe' means the Universe seen from our vantage point in the Milky Way Galaxy.
The Universe is huge and possibly infinite in volume. The matter which can be seen is spread over a space at least 93 billion light years across. For comparison, the diameter of a typical galaxy is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years. As an example, our Milky Way Galaxy is roughly 100,000 light years in diameter, and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away. The observable Universe contains more than 2 trillion (10) galaxies and, overall, as many as an estimated stars (more stars than all the grains of sand on planet Earth).
Typical galaxies range from dwarf galaxies with as few as ten million (10) stars up to giants with one trillion (10) stars, all orbiting the galaxy's center of mass. Thus, a very rough estimate from these numbers would suggest there are around one sextillion (10) stars in the observable Universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion (7 x 10).
The matter that can be seen is spread throughout the Universe when averaged over distances longer than 300 million light-years. However, on smaller length-scales, matter is observed to form 'clumps', many atoms are condensed into stars, most stars into galaxies, most galaxies into galaxy groups and clusters and, lastly, the largest-scale structures such as the Great Wall of galaxies.
The present overall density of the Universe is very low, roughly 9.9 × 10 grams per cubic centimetre. This mass-energy appears to consist of 73% dark energy, 23% cold dark matter and 4% ordinary matter. The density of atoms is about a single hydrogen atom for every four cubic meters of volume. The properties of dark energy and dark matter are not known. Dark matter slows the expansion of the Universe. Dark energy makes its expansion faster.
The Universe is old, and changing. The best good guess of the Universe's age is 13.798±0.037 billion years old, based on what was seen of the cosmic microwave background radiation. Independent estimates (based on measurements such as radioactive dating) agree, although they are less precise, ranging from 11–20 billion years.
to 13–15 billion years.
The Universe has not been the same at all times in its history. This getting bigger accounts for how Earth-bound people can see the light from a galaxy 30 billion light-years away, even if that light has traveled for only 13 billion years; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; the photons emitted have been stretched to longer wavelengths and lower frequency during their journey. The rate of this spatial expansion is accelerating, based on studies of Type Ia supernovae and other data.
The relative amounts of different chemical elements — especially the lightest atoms such as hydrogen, deuterium and helium — seem to be identical in all of the Universe and throughout all of the history of it that we know of. The Universe seems to have much more matter than antimatter. The Universe appears to have no net electric charge. Gravity is the dominant interaction at cosmological distances. The Universe also seems to have no net momentum or angular momentum. The absence of net charge and momentum is expected if the Universe is finite.
The Universe appears to have a smooth space-time continuum made of three spatial dimensions and one temporal (time) dimension. On the average, space is very nearly flat (close to zero curvature), meaning that Euclidean geometry is experimentally true with high accuracy throughout most of the Universe. However, the Universe may have more dimensions, and its spacetime may have a multiply connected global topology.
The Universe has the same physical laws and physical constants throughout. According to the prevailing Standard Model of physics, all matter is composed of three generations of leptons and quarks, both of which are fermions. These elementary particles interact via at most three fundamental interactions: the electroweak interaction which includes electromagnetism and the weak nuclear force; the strong nuclear force described by quantum chromodynamics; and gravity, which is best described at present by general relativity.
Special relativity holds in all the Universe in local space and time. Otherwise, general relativity holds. There is no explanation for the particular values that physical constants appear to have throughout our Universe, such as Planck's constant "h" or the gravitational constant "G". Several conservation laws have been identified, such as the conservation of charge, conservation of momentum, conservation of angular momentum and conservation of energy.
Accurate predictions of the Universe's past and future require an accurate theory of gravitation. The best theory available is Albert Einstein's general theory of relativity, which has passed all experimental tests so far. However, since rigorous experiments have not been carried out on "cosmological" length scales, general relativity could conceivably be inaccurate. Nevertheless, its predictions appear to be consistent with observations, so there is no reason to adopt another theory.
General relativity provides of a set of ten nonlinear partial differential equations for the spacetime metric (Einstein's field equations) that must be solved from the distribution of mass-energy and momentum throughout the Universe. Since these are unknown in exact detail, cosmological models have been based on the cosmological principle, which states that the Universe is homogeneous and isotropic. In effect, this principle asserts that the gravitational effects of the various galaxies making up the Universe are equivalent to those of a fine dust distributed uniformly throughout the Universe with the same average density. The assumption of a uniform dust makes it easy to solve Einstein's field equations and predict the past and future of the Universe on cosmological time scales.
Einstein's field equations include a cosmological constant (Lamda: "Λ"), that is related to an energy density of empty space. Depending on its sign, the cosmological constant can either slow (negative "Λ") or accelerate (positive "Λ") the expansion of the Universe. Although many scientists, including Einstein, had speculated that "Λ" was zero, recent astronomical observations of type Ia supernovae have detected a large amount of dark energy that is accelerating the Universe's expansion. Preliminary studies suggest that this dark energy is related to a positive "Λ", although alternative theories cannot be ruled out as yet.