Scientists first make observations that raise a particular question. In order to explain the observed phenomenon, they develop a number of possible explanations, or hypotheses. This is the inductive part of science, observing and constructing plausible arguments for why an event occurred.
As indicated earlier, the Bible does not fix the age of the earth, contrary to the claims of Answers in Genesis. Historically, their claim comes from the work of James Ussher, Bishop in the Church of Ireland, from 1625 to 1656.
Furthermore, they are generally not aware that the big bang contradicts the Bible on a number of points and has many scientific problems as well. According to the big bang idea, the universe is nearly 14 billion years old; whereas the Bible indicates that the universe is about 6,000 years old. For those who claim to believe the Bible, this difference alone should be sufficient reason to reject the big bang.