One of the (rather many) unsolved problems of physics is that there are certain constants that cannot be derived from something else - they can only be measured. They just ARE. One of these is the fine structure constant, the coupling constant for the electromagnetic interaction (it’s value is about 1/137). Several others include the speed of light c, vacuum permittivity ε0, Planck's constant h, and the gravitational constant G. There are supposedly 26 known fundamental dimensionless physical constants, but some of them require that you believe in the current approximation of a Theory of Everything (“TOE”), called the Standard Model of particle physics. It’s not a real TOE, of course, because for nearly a century now no one has been able to figure out how to meld gravity with quantum mechanics. The theory du jour (for the past 30+ years) for this is actually a melange called String Theory, which requires you to believe that there are 10 or 11 dimensions in our universe. One must exercise a lot of faith to be a physicist these days.
Here’s the interesting thing about these otherwise unexplainable constants: if ANY one of them were just slightly different, no life could exist in the universe.
One example may be helpful here: the fine-structure constant, governing the four fundamental interactions (gravity, electromagnetism, the “weak” nuclear force, and the “strong” nuclear force) is so finely-tuned that it allows the formation of commonly found (baryonic) matter, and thus the emergence of life. A fraction of a percent increase in the strong nuclear force would bind the dineutron and the diproton, and nuclear fusion would have long ago converted all hydrogen in the early universe to helium. Water, organic chemistry, and stars essential for the emergence of life would not exist. This amazing - even incredible - coincidence is called the anthropic principle. Another way to say this is that these finely-tuned physical parameters are a necessity: because living observers wouldn't be able to exist, and thus observe the Universe, if these constants were not just precisely what they are. Physicists have no idea why this is the case - it just is.
Even the current age of the universe is critical: if it were a fraction of the current age, there would not have been sufficient time to build the heavier elements (especially carbon and oxygen) from novas and supernovas. There would be no rocky worlds with salty seas to harbor life.
Attempts to explain the anthropic principle - this precise tuning that permits life to exist - invoke either of two ideas: the existence of multiple universes (the “multiverse”), or an Intelligent Designer. Why a multiverse? So that there can be an infinite number of universes including the ONE that harbors life... so we exist by a sort of cosmic natural selection. There are scientists and philosophers who have problems with both explanations, and of the two, a multiverse is completely untestable. Some critics conclude that the anthropic principle is more of a philosophical concept, not a scientific principle. One way some scientists have tried to bypass the controversy is to emphasize the weak anthropic principle, i.e., the conditions that we observe in the universe must permit the observer to exist. In mathematics and philosophy, the weak form of an argument is one which is easier to support - it makes fewer claims.
It's certainly weak. In fact, if you find all these arguments either non-explanations or circular reasoning, then put a gold star on your forehead. I personally find it amazing that some scientists are so desperate to avoid acknowledging the existence of a Designer for this universe, that they will flail around trying to promote something as untestable as String Theory, or a multiverse. These are adults - with PhD's!
So, who is the believer in something they can’t see here?