The term 'Big Bang' refers really to the general model that this Universe had a beginning in time (and is expanding, as we can see it doing right now, by observation),
instead of one of the competing theories like the oscillation or even the old steady state etc., the competing theories where this Universe did
not have a beginning.
Really,
if you think this Universe had a beginning you already are accepting the central most key part of the 'big bang' model.
Belief in a Beginning = belief in an initial coming into existence.
Now, under the tent of the 'Big Bang' general model we currently have also the theory of 'cosmic inflation', so that there is not only a beginning, but also there is a faster than light speed expansion so that the basically from-nothing Universe expands in an instant so brief, so vastly much less than 1 second, that it takes powers of ten notation just to state that time period. To a human perception, unable to perceive below the level of something like a sound as brief as 0.0002 milliseconds of time, if we could somehow watch it happen (but from
where???
), it would basically just suddenly be in existence.
And at first incomparably bright and we wouldn't be able to look at that level of brightness with these eyes. (and then in theory later it would be mostly dark, awaiting the light from the first stars). So this is theory based on observation actually -- the 'cosmic background radiation' -- which, believe it or not (and I'm not concerned with whether you will or won't believe these theories, though you can certainly check on the
observations), is literally the left over afterglow of the early Universe.
It's from observations these theories arose, see.
So, it's valuable to notice the big difference between physics -- the basic science of things like energy, radiation, gravitation, magnetism, elementary particles, and so on -- and the very different science, vastly different, of biology, which isn't a study of elementary particles and forces like physics is, but is instead a study of complex systems that cannot be calculated from physics, due to the immense calculation that would be required.
They are 2 extremely unalike types of science in this key way, one can see.
But still, generally those trying to figure out 'evolution' are indeed trying to explain observations of fossils, for instance, so they are doing a type of science, still, even though it is far less testable or reproducible than most of physics. In other words, both are science, though one can argue physics is by nature far more tested and solid.
Note though that one thing we know in physics is that radioactive elements have 'half lives' where 1/2 of the material will decay into another isotope or element, emitting radiation as it decays. That's not only theory, but direct observation. From this solid fact, we can then begin the careful work of figuring out how old many things are, and over time, with much effort, get more and more reliable estimates of ages since a rock or fossil formed, using the ratios of isotopes in that material.