The constants of physics are exquisitely tuned
The claim
Roughly 30 dimensionless constants must lie within absurdly narrow ranges for any complex chemistry, any stable matter, any observers at all.
Three explanations remain viable: brute coincidence (a probability vanishingly small that we are forbidden from being puzzled by), an unobservable multiverse postulating ~10^500 universes to explain one, or design. Design is by far the cleanest.
The numbers are too good to be coincidence.
The cosmological constant agrees with what life requires to one part in 10^120. The ratio of the strong to the electromagnetic force, the proton-to-electron mass, the early-universe entropy: each must be threaded simultaneously through a needle.
Theism predicts this. Naturalism does not. The multiverse is naturalism's attempt to make naturalism predict it, by postulating enough trials to make the long-shot likely. The cost is metaphysical: an infinity of unobservable worlds.
Between two explanations equally compatible with the data, the simpler is preferred. One God or 10^500 universes — the choice is not difficult.