The Goldilocks effect
Those unfamiliar with astrophysics might get the impression that the Big Bang was just a random explosion of energy that just happened to produce galaxies with stars and at least one planet capable of supporting intelligent life. But nothing could be further from the truth.
The more physicists have learned about the conditions for a stable universe, and in particular a universe capable of sustaining intelligent life, the more it seems that the Big Bang must have been very finely tuned. Like Goldilocks's porridge, the universe had to be just right.
Example: gravitational force
An example of fine-tuning is the strength of gravitational force. If gravitational force were too strong then matter would clump together, if gravitational force were too weak then bounds between particles would be too weak. In either case, stars like our sun could not have formed and without the sun, life on planet could not exist. But what is really surprising is just how particular fine tuning is. If the strength of gravitational force had differed by one part in 1040 then our sun could not exist. (1040 is scientific notation for a 1 followed by 40 zeroes, or in other words, ten thousand billion billion billion billion.)
This is just one example of many conditions that are remarkably finely tuned. Other examples include the difference in mass between a proton and neutron, and the density of the universe.
Crying out for an explanation
The point about these examples is not simply that they are improbable, but that they are crying out for an explanation. Imagine if you replayed the Big Bang over and over again, billions upon billions of times. And imagine that each time there was a Big Bang, you changed one of starting conditions (say, gravitational force) by a small degree. In almost every case the universe that emerged would either quickly collapse in on itself or would be entirely made up of hydrogen and helium; the scenarios under which the Big Bang produced a universe capable of sustaining intelligent life would be a tiny tiny percentage. This specified complexity requires an explanation and for a lot of people that explanation is a Designer.
These examples of fine-tuning are not controversial. The physicist Paul Davies has written, "Everyone agrees that the universe looks as if it was designed for life." Believers and non-believers agree that these remarkable coincidences require an explanation. However, there have been some attempts to propose an explanation that doesn't require a Designer. Perhaps the most common alternative is the multiverse explanation, whereby there just are billions upon billions of universes and eventually one of them would turn out to be like ours. It is questionable whether this is a better explanation. Firstly, the multiverse is entirely theoretical and it is not clear how one might go about trying to prove it. Secondly, it seems odd to choose to hypothesise billions upon billions of universes just to escape the existence of one God. Thirdly, the multiverse hypothesis seems to complicate, not simplify, the fine-tuning, as now one has to explain the origin of billions upon billions of universes.
God is the most straightforward explanation of the fine-tuning of the universe.