The universe is a unity composed of space, time, matter and energy. It is the sum total of all space and time. The universe as generally understood refers to a space-time continuous system in which we exist, including all matter, energy and events within it. The overall explanation of this system constitutes the cosmology. In recent centuries, scientists have developed a modern scientific theory about the universe, called physical cosmology, based on modern physics and astronomy. According to the theory of relativity, the propagation speed of information is limited, so in some cases, such as the expansion of the universe, we will only receive information from a small part of the area very far away from us, and the information from other parts will It could never spread to our area. The part of space-time that can be observed by us is called the "observable universe", the "visible universe" or "our universe". It should be emphasized that this is due to the structure of space-time itself and has nothing to do with the observation equipment we use. The universe is composed of approximately 4% ordinary matter (including us humans and the earth), 23% dark matter, and 73% dark energy.