When I was a boy with a deep interest in astronomy, there were three alternative scientific theories to explain the origin of the universe and why it is expanding. Two of them (Steady State and Pulsating universe) were based on the idea that the universe has always existed, and one of them that it began a definite time ago.
In the years since then, the latter theory, the Big Bang, has become almost universally accepted by scientists, who believe the evidence indicates that the universe, matter, energy, 3-dimensional space and time all began around 15 billion years ago.
This conclusion is based on the fact that the universe is expanding, with the more distant galaxies moving faster, so we can extrapolate backwards to the time when it existed as a single point, or singularity. The big bang also explains a number of other cosmological discoveries, regarding temperature and background radiation, for example.
The big bang was not an explosion in empty space. Rather, the big bang created space - at the moment of the big bang, 3-dimensional space and the entire universe were infinitesimally small, but began to expand rapidly. And we don't know if there was anything before the big bang, because time was also created at the big bang.
If the universe has always existed (as was once believed), we wouldn't need to ponder the cause, because if it didn't have a start, it presumably didn't have a cause. But the big bang almost demands an explanation. Most scientists consider it unlikely that we can ever discover scientifically what caused the big bang, but of course philosophers, theologians, scientists (and nearly everyone else) still wonder.
Did something cause the big bang, and if so, what was it?
There are only a few options ......
- There was no cause.
- There was a "natural" cause.
- There was a non-natural ("supernatural") cause.
The arguments (summarised) go something like this ....
- The idea there was no cause, or that the universe caused itself, goes against everything we know about things - everything in the universe has a cause outside itself. But we don't have much experience of big bangs (this is the only one we know about) so perhaps it is different to everything else we know and doesn't need a cause.
- A natural cause is something measurable by science. But if the universe had a natural cause (say another universe that caused our universe to form), we have just pushed the question back another stage, as we then ask what caused that universe or cause. Philosophers and scientists are quite sure that an infinite series of causes cannot occur and doesn't make sense, so this explanation must at some stage end up in either option 1 or option 3.
- A supernatural cause, such as a god, does not suffer from this difficulty if one believes that god has always existed - like a universe that has always existed, a god that has always existed does not require a cause. Of course if one accepts this explanation, it does not demonstrate which particular god might have created the universe, although many of the gods believed by people would not be capable of such a thing. (For example, the gods of Norse mythology were part of the universe, not creators of it.)
So it it appears to come back to two options - either the universe did not have a cause, it just happened, or it had an eternal, supernatural cause, which might be defined as "god".
Even with all the understanding of modern cosmology, it seems we still come back to the same age-old questions and answers.