There are several pieces of evidence that astrophysicists use to support the Big Bang theory. Let us briefly look at some of the bigger pieces of evidence for the theory.
Through observations made with microwave telescopes, we can see something known as background radiation. This term describes perhaps the biggest piece of evidence for the Big Bang theory. Essentially, background radiation consists of the photons emitted by free electrons before they could form atoms. These free electrons were scattered around the universe after the Big Bang.
Deep space surveys have also found an interesting phenomenon known as inflation. These surveys found distant regions of space that have the same temperature. This raised the question of how and why places many light-years apart could be so similar in this respect. The Big Bang answers this question. If the universe was once extremely small before a short period of quick expansion, these now distant areas may have once shared the same source of energy that imparted them with properties that gave them the same temperature.
The redshift refers to what is happening to the observed light of distant galaxies. Light from stars in these galaxies is shifting red over time. This particular Doppler effect indicates that galaxies are moving away from one another. The furthest ones are moving the fastest. This indicates that everything in the universe is moving away from everything else, as would happen after the Big Bang.
The Big Bang still remains a theory. There are still some elements of it that are not fully understood. However, when this evidence is considered together, it supports the main principles of the Big Bang.