Scientists have come to understand that there are measurable features of our universe that make life possible. These are often discussed in scientific and philosophical works as variations on the anthropic principle. Christian apologists sometimes refer to it as the argument for God from the fine-tuning of the universe.
It may be helpful to think of the fine-tuning question at several levels. At the most fundamental level, we look at the physical constants of the universe necessary for the universe to exist at all. At a second level, given the existence of the universe, there are constants necessary for life. At a third level, there are attributes particular to earth, that make it a planet particularly hospitable to life.
In the course of learning more and more about the universe in which we live, scientists have come to understand that aside from matter, there are certain key forces at work upon that matter, including gravity, electromagnetism, and strong and weak nuclear forces. It turns out that there is an extremely narrow band of tolerance for these forces. For example, if the force of gravity were ever-so-slightly different than what it is, no universe would be possible at all.
That the force of gravity (using this one example of force) is so perfectly “tuned” to allow for the existence of matter suggests that there was a Mind which established that force with such precision. If the force of gravity were mathematically tweaked just the slightest bit, we would end up not with a different kind of universe, but with no universe at all. A slight variation in the force of gravity is the difference between physical nothing, and something.
Atheists would argue (or would have to argue, to be consistent) that the force of gravity is not finely-tuned at all, because to be finely-tuned, there would have to be One to do the tuning. Gravity, they would have to say, just is what it is, and what it is (along with other cosmological constants) happens to result in a universe.
This appeal to happenstance as an explanation for material existence ought to be doubly troubling, for atheists, because in their paradigm, material existence is all of reality. But for this precise measure of gravitational force extant in the universe, there would be absolute nothing. Not merely a different kind of universe. We’re talking about all of material existence. And that means, for an atheist, all of reality.
The fact of something rather than nothing is dependent on just this particular value for gravitational force, and so it sure begins to look like the gravitational force we have was purposely designed. There would have to be some intelligent and purposeful Entity; some active agency, to do the designing. “God,” to give it a name.