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What is Universe ?

The universe is all the matter, energy, and space that exist. We can observe only a part of it - the observable universe. The entire, universe, including the part we cannot see, may be infinite.

 Galaxies form and evolve within an expanding and cooling universe. Because light takes billions of years to cross the cosmos, we see distant galaxies as they were at earlier times. Astronomers not only theorize about cosmic evolution - they can actually observe its various stages.
The Big Bang is the scientific description of the origin of the cosmos. It describes the beginning of the universe as an explosion of space, which has expanded and cooled ever since.
If we rewind the expansion of space and go backward in time, galaxies crowd closer and closer together in the past. Ultimately, all matter and energy is compressed at extremely high density and temperature. The observable universe fits within a volume smaller than a grain of sand. That superdense medium exploded in the Big Bang, launching the expansion of the universe.
The Big Bang theory makes three major predictions that have been tested and confirmed by observational evidence.



YouTube Link - https://www.youtube.com/watch?v=gIr9TQnV25A

Credits - https://www.amnh.org/exhibitions/permanent-exhibitions/rose-center-for-earth-and-space/dorothy-and-lewis-b.-cullman-hall-of-the-universe/the-universe/the-big-bang

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