How can the Big bang theory have overwhelming evidence when no one was around to observe or measure it, which are required in the scientific method? 

'Grok' answers.

The Big Bang theory, like many scientific theories, doesn't require direct observation of the event itself to be considered well-supported by evidence. Here's how this works within the scientific method:

Indirect Observation:
Science often deals with phenomena that cannot be directly observed. For instance, we don't see atoms with the naked eye, yet we have substantial evidence for their existence through indirect methods. Similarly, the Big Bang is supported by indirect observations of its consequences or "fossil" evidence.
Predictive Power:
The scientific method values the ability of a theory to make predictions about what we should observe if the theory is correct. The Big Bang theory has made numerous predictions:
Cosmic Microwave Background Radiation (CMB): Predicted before its discovery, the CMB's characteristics match Big Bang predictions.
Element Abundance: The theory accurately predicted the ratios of hydrogen, helium, and other light elements formed shortly after the Big Bang.
Universe Expansion: Predicted that the universe would be expanding, which was later confirmed by observations of redshift in galaxy light.
Consistency with Known Physics:
The Big Bang model integrates with well-tested physical laws, including general relativity, quantum mechanics, and thermodynamics. It explains how these laws could govern the behavior of the universe from its earliest moments.
Falsifiability:
A key aspect of the scientific method is that theories must be falsifiable. If observations were found that contradicted the Big Bang model's predictions, the theory would need revision or rejection. So far, no such contradictory evidence has been found; instead, each new piece of data fits into the Big Bang framework or extends our understanding.
Multiple Lines of Evidence:
The Big Bang theory isn't based on a single piece of evidence but a confluence of many:
Redshift of galaxies indicating expansion.
Cosmic background radiation as a remnant of the early universe.
Large Scale Structure of the universe matching simulations from Big Bang models.
Nucleosynthesis predictions about light element abundances.
Ockham's Razor:
While not a formal part of the scientific method, this principle suggests that, among competing hypotheses, the one with the fewest assumptions should be selected. The Big Bang theory explains a wide range of observations with relatively few assumptions compared to other cosmological models.
Historical Analogy:
Much like how geologists infer past geological events from current rock formations or paleontologists infer past life from fossils, cosmologists infer the universe's past from the "fossils" of the Big Bang.

In essence, while no one was there to "see" the Big Bang, the scientific method allows us to test theories through their implications and predictions in the present. The overwhelming evidence for the Big Bang is not from direct measurement of the event but from the coherence and predictive success of the theory in explaining current observations about the cosmos.