The Big Bang theory is a scientific theory, not a scientific fact. In science, a theory is a well-substantiated explanation of some aspect of the natural world that is based on a large body of evidence and has withstood rigorous testing and scrutiny. Theories are the highest level of confidence in scientific understanding and are supported by a wide range of observations, experiments, and mathematical models.
Calling the Big Bang theory a "fact" is a colloquial use of the term and can be misleading. It is more accurate to refer to it as an extensively supported scientific theory. However, the term "fact" is sometimes used informally to describe scientific theories that have overwhelming evidence supporting them. In this sense, the Big Bang theory is often referred to as a fact because it is supported by a vast amount of observational and experimental evidence.
The Big Bang theory explains the observed expansion of the universe, the abundance of light elements, the cosmic microwave background radiation, and various other phenomena. It has been tested and confirmed by numerous experiments and observations, including measurements of the cosmic microwave background radiation, the distribution of galaxies, and the redshift of distant objects.
Despite its strong empirical support, scientific theories are always subject to revision or refinement in light of new evidence or more comprehensive theories. This is the nature of scientific progress. The Big Bang theory continues to be studied and refined, and scientists actively investigate questions related to its early stages and potential extensions to our current understanding.