Why does the universe and everything in it exist? In other words: Why is there something instead of nothing?
Scientists say that all the matter in the universe is ultimately made of energy, and that matter and energy can change back and forth into each other. But how did all the matter and energy in the universe originally come into existence? Such a question takes on special meaning when you realize that most scientists accept as a Scientific Law1 that in no situation can any matter/energy cease to exist, and in no situation can any matter/energy come into existence out of nothing. To put it another way:
Something cannot change into nothing, and nothing cannot change into something.
This widely accepted Scientific Law infers that the combined total of all matter and energy in the universe cannot change, and therefore has always been the same.
Scientists have never reached a consensus on why there is something instead of nothing. Several theories have been proposed, but they all fit into one of the following categories:
1) The scientific law mentioned above was violated one or more times in the past (i.e. something came from nothing).
2) Matter and/or energy is eternal, i.e. it had no beginning. It has always existed.
3) Some combination of (1) and (2).
4) An eternal God (or gods) created all matter, energy, and space2 from nothing.
With only these four proposals to currently choose from, it is not surprising that scientists have been unable to reach a consensus on why there is something instead of nothing.
References and Notes for Chapter 16:
1. First Law of Thermodynamics. See Chapter 14: Did the Big Bang Create Anything?
2. See Chapter 15: Is Empty Space the Same as Nothing?