The physical constants governing the universe — the strength of gravity, the electromagnetic force, the strong and weak nuclear forces, the cosmological constant, and dozens of others — are calibrated to values so precise that even infinitesimal changes would render the universe lifeless.
The gravitational constant is tuned to approximately 1 part in 10^36. If gravity were slightly stronger, stars would burn too hot and too quickly for life to develop. If slightly weaker, stars would never ignite and no heavy elements would form. The strong nuclear force is tuned to within 2%.
The cosmological constant — the energy density of empty space — is perhaps the most striking example. Its observed value is approximately 10^-122 in natural units. If it were even slightly larger, the universe would have expanded too rapidly for any structures to form. If slightly negative, the universe would have collapsed back on itself.
The ratio of electrons to protons is balanced to 1 part in 10^37. The expansion rate of the early universe was fine-tuned to 1 part in 10^55. The list extends to over 30 independently tuned constants.
Physicist Paul Davies summarized the situation: the impression of design is overwhelming. Astronomer Fred Hoyle, an atheist, famously remarked that it appeared as though a superintellect had monkeyed with physics.
Three main responses exist: (1) Design — the constants were set by an intelligent creator. (2) Multiverse — there are countless universes with different constants, and we observe ours because we could only exist in one fine-tuned for life. (3) Necessity — perhaps a future theory of everything will show the constants could not have been otherwise.