If the strong nuclear force were just 2% greater in magnitude, hydrogen not undergo the normal process of fusion into helium, but rather create stable diprotons, meaning that stars as we know them would not exist. This narrowness of the range of physical constants controlling the process is an example of fine tuning. First precisely described by Dicke in the 1960's fine-tuning problems arise in the value of the cosmological constant, the ratio of gravitational to electromagnetic force strengths, the abundance of dark matter in the universe and the curvature of the space (see flatness problem).
Questions of fine tuning have been addressed in different ways: Some argue that the anthropic principle provides an answer - that we could not ever observe parameters incompatible with our existence. It is also possible that the observed constants are the consequence of a larger theory, and thus not arrived at by chance. Finally some argue that in multiverse scenarios all values of these constants exist somewhere therefore it is no surprise to find ourselves inhabiting such a region.