Big Bang
Scientists base the Big Bang theory on many different observations. The most important is the redshift of very far away galaxies. Cosmological redshift is the Doppler effect occurring in light. When an object moves away from Earth, its color rays look more similar to the color red than they actually are, because the movement stretches the wavelength of light given off by the object. Scientists use the word "red hot" to describe this stretched light wave because red is the longest wavelength on the visible spectrum. The more redshift there is, the faster the object is moving away. By measuring the redshift, scientists proved that the universe is expanding, and they can work out how fast the object is moving away from the Earth. With very exact observation and measurements, scientists believe that the universe was a singularity approximately 13.8 billion years ago. Because most things become colder as they expand, scientists assume that the universe was very small and very hot when it started. This does not take into account other possible (non-cosmological)  causes of redshift.