In a footnote to one of his articles, creationist Gary DeMar (2007) seems to be critical of How was the Big Bang possible if something cannot come from nothing? After dismissing the Big Bang as speculation, he quotes from my article in the footnote and asks the reader to notice that I explain Big Bang theory by appealing to other theories (general relativity and quantum mechanics). His implication presumably is that scientific theories are mere conjecture.
The short response
General relativity and quantum theory are theories in the scientific sense of the word, not in the colloquial sense. They are not speculative. You benefit from technology that depends upon their accuracy.
The longer response
The fact that scientists call general relativity a theory does not make it speculative. On the contrary, general relativity is as well-confirmed as anything can be. Forget gravitational lensing and the detection of gravitational waves—at a more mundane level, every time you depend on GPS or someone who uses it you depend on the accuracy of the general theory of relativity. Likewise for quantum mechanics: no quantum mechanics, no microprocessors.
To dismiss general relativity or quantum mechanics as speculative because scientists call them theories is to make same mistake that creationists make when they dismiss evolution as “just a theory”: conflation of the colloquial sense of a word with the technical sense of the word as used by specialists. Scientists do not use the word theory to refer to speculation; in scientific circles, a theory is a wide explanatory framework that has reached the highest standard of confirmation by evidence.
lf you are interested in a more thorough excursion through the evidence for general relativity, Astronomy Notes presents a fine overview.
DeMar G. 2007. Why evolution is impossible. https://americanvision.org/1087/why-evolution-impossible/