A simple reflection tells us that something must be eternal. After all, if you start with nothing, you’ll always end up with nothing. But we ended up with something, which means we must have started with something. Put another way, since something exists now something must have always existed. There could never be a time when absolutely nothing existed. Something must be eternal, but what is that something?
There are good scientific and philosophical reasons to conclude that physical reality has not always existed, so physical reality can’t be the eternal something. Since things which begin to exist must be caused to exist by something else, physical reality had to be caused by something else, and perhaps the cause of the physical world is the eternal something we are looking for. How would we know?