The galaxies our telescopes can detect contain about 100 billion billion stars. The Big Bang Theory was conceptualized to explain these two primary scientific observations, i.e., elemental abundance and galactic recession. This behavior is exactly what would result from an explosion of matter compacted at a single point in the universe sometime in the past. In fact, by running the recession rate backwards, we can calculate how many years it has been since all of the galaxies were in the same place. This turns out to be 15 billion years ago.
Most good theories raise other intriguing questions.
We do not know. We do not know. One problem faced by advocates of Big Crunch is that the calculated mass of all the galaxies we can see is too small to exert the necessary gravitational attraction. The expansion of the blast was so great, that density and temperature fell off too rapidly to permit further collisions of fundamental particles. After 100,000 years, electrons were captured releasing a great flash of light, the remnants of which are still observable today in the microwave region.