the FLRW model of cosmology, which has made numerous confirmed predictions teaches us that the big bang is a geometric property of space-time i.e. a consequence gravity, and particularly Einstein's field equation from general relativity.
It's interesting to put that in perspective: the pyramids were built out of solar energy by way of muscle. Skyscrapers are built out of solar energy mostly by way of diesel fuel.
It is. Except for geothermal energy brought on by radioactive decay, all energy on earth originally comes from the sun. Fossil fuel is sunlight that was absorbed by plants which then either decomposed into fuel over millions of years, or were eaten by animals which later decomposed into fuel over millions of years. Our cars are powered by very old, stored sunlight.