Cosmology, the late 20th century, are classified into two main branches: experimental (experimental cosmology) and theory (cosmology theory). The experimental cosmology was almost abandoned hope of the entire observable universe, while, the cosmology theory has developed models for the entire universe, despite possible feature of this theory will not have enough empirical evidence to test.

Words "observable universe", "see the universe" is for the universe that people can feel through empirical means.
Milestone in the history of space exploration by human beings:

- 4th century BC, Aristarchus seems to be the first to learn about planetary systems and the heliocentric system. Discovery as opposed to the concept of the world of Aristotle. He has quite accurately calculated the distance between Earth and Moon.
- 3rd century BC, Eratosthenes also calculate the Earth's circumference at the equator only differences about 650 km (error 1.5%).
- 2nd century BC, Hipparchus calculating the distance between Earth and Moon, 1500 star statistics, approximation of the precession cycle of the Earth's axis.
This knowledge of the Arabs again. When the Arabs expanded influence, book Almagest of Ptolemy the geocentric universe model is spreading.
Copernican revolution overturning the geocentric conception Left:
Nicolaus Copernicus rediscover heliocentrism.
Isaac Newton and Johannes Kepler determined the motions of the planets, based on the laws of Newton and discover gravity, which is verified by astronomical observations.
Giordano Bruno applied the solar system for all other stars, the universe expanded extremely.
The technical progress of astronomy in the 20th century opened up a series of discoveries of strange objects in the universe (like the stars in different stages of development), the macro structure of Universe head (including the stars gathered in galaxies and galaxy groups), and especially the trend expansion of the universe, observations by Edwin Hubble.

Observations about the expansion of the universe is one of the prerequisites to build their models on the evolution of the universe is the most widely recognized today, Big Bang model.