The universe is everything. It is all of space and time and everything contained within it, such as galaxies, planets, moons, asteroids.
The universe is everything, all space, matter, and time itself. Scientists are still unsure how big the universe is, but they believe that it is roughly 13.8 billion years old. The word comes from the Latin universum meaning "all things". Astronomers used to think that the universe was entirely composed of matter made of atoms. There is now an increasing amount of evidence to suggest that there is a lot of matter that we don’t understand as well, known as dark matter.
As time has progressed, ideas about the structure of the universe have changed. The earliest model put the Earth at the center of the universe. Later Nicolaus Copernicus developed another model of the universe that put the Sun at the center. Copernicus’ ideas were supported by observations of the moons of Jupiter. As time passed and using better telescopes we now know that Earth’s position in the universe is not in any way special. Earth is one of a huge number number of planets, which are orbiting a huge number of stars in a huge number of galaxies.
The leading theory of how the universe started is a theory called the Big Bang theory. The term "Big Bang" is a famous misnomer. The theory says that the universe started as a very small, dense point and expanded from there. The current evidence of redshift and the existence of cosmic microwave background radiation supports the Big Bang theory. Redshift is the observation that the universe is expanding. When scientists observe distant galaxies, the light they observe is redder than they would normally expect. This change is similar to the Doppler effect where waves that are moving relative to the orbiter have longer wavelengths.
Scientists can observe the universe in a number of different ways. We have sent spacecraft to visit and explore different planets and moons within our solar system. There are also observatories with telescopes all over the world and in space. The Hubble space telescope has been an extremely useful tool to observe and study the nature of the universe.