the Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. Based on observations of the portion of The Universe that is observable, physicists attempt to describe the whole of space-time, including all matter and energy and events which occur, as a single system corresponding to a mathematical model.
The currently accepted theory of the Universe's formation is the Big Bang model, which describes the expansion of space-time from a gravitational singularity. The Universe underwent a rapid period of cosmic inflation that flattened out nearly all initial irregularities. Thereafter the universe expanded and became steadily cooler and less dense. Minor variations in the distribution of mass resulted in fractal segregation into features that are found in the current universe; such as clusters of galaxies. There are probably more than one hundred billion galaxies in The universe.
Theoretical and observational cosmologists vary in their usage of the term Universe to mean either this whole system or just a part of this system.
As used by observational cosmologists, the Universe (upper case "U") most frequently refers to the finite part of space-time which is directly observable by making observations using telescopes and other detectors and using the methods of theoretical and empirical physics for studying the basic components of The Universe and their interactions. Physical cosmologists assume that the observable part of (comoving) space (also called: "our universe") corresponds to a part of a model of the whole of space, and usually not to the whole space. They frequently use the term the universe to mean either the observable part of space, the observable part of space-time or the entire space-time.
A majority of cosmologists believe that the observable universe is an extremely tiny part of the "whole" (theoretical) Universe and that it is impossible to observe the whole of comoving space. It is presently unknown whether or not this is correct, since according to studies of the shape of the Universe, it is possible that the observable universe is of nearly the same size as the whole of space, but the question remains under debate. If a version of the cosmic inflation scenario is correct, then there is no known way to determine whether the (theoretical) universe is finite or infinite, in which case the observable Universe is just a tiny speck of the (theoretical) universe.
Source: www.wikipedia.com
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