They then design experiments to test their theories and base their conclusions on those experiments.
If a scientific conclusion is found to hold up under repeatable experimental conditions over a long period of time, it is held to be true.
If the experimental evidence does not support a theory, science goes back to the drawing board and starts again.This process has worked very well in helping us to understand all aspects of the universe over the whole of human existence, as well a very strong case for evolution by the way.
As regards Big Bang, by no means do scientists assert this theory is 'definitely true' but it does currently best fit the evidence over many decades, which we continue to gather and analyse.
Personally, I prefer the scientific route to understanding the universe. By all means pick the Bible as a literally true text if you wish, but please also understand that the Bible is at odds with evidence when you do so, and that is why most of us reject it as a true depiction of what we see around us.
As for the 'waste of time and effort' alluded to by Mr Johnston, theoretical physics research has given us computers, the whole telecommunication revolution, and the internet just for starters.
David Robinson
Address supplied

In response to William Johnston's letter in last week's journal asking for 'undeniable proof ' of evolution, allow me to oblige: Dinosaurs.
H. Broad
Grantham