What is evidence that scientist use to support the big bang theory?

One big observational evidence that scientists have is the Cosmic Microwave Background Radiation (CMBR). This evidence is the most accurate because astronomers have claimed that the universe is evolving a set amount of matter and energy while it is expanding over time.

The Big Bang theory is among the most widely accepted today to explain the origin of the universe. It holds that the universe emerged from the explosion of a single particle - the primordial atom - causing an unparalleled cosmic cataclysm about 13.8 billion years ago. The same theory further states that the universe is continually expanding. The main evidence for the big bang theory is:

The galaxies to get further and further from us. This distance is taken at high speeds. In short, the spacing velocity of the galaxies is proportional to the distance these galaxies actually are. This evidence is called the "Hubble Law" and explains that our impression of galaxy dysthanization occurs because the universe is still expanding, and this theory further explains that the universe has already been connected. The big bang theory states that the universe was already very hot, if this is true, we should find some evidence of this temperature. Well, this evidence was found when radio astronomers realized that there was cosmic background radiation in a microwave that had a temperature of 2,725 degrees Kelvin. This radiation is likely to be evidence of the high temperature the universe had. Finally, the abundance of hydrogen and helium elements probably resulted from the Big Bang.