When we look at the vastness of the Universe - all those distant galaxies and stars and other things that we’ll never see in our life time, we have the urge to throw up our hands and say all of this is beyond explanation and has always existed. This is what the Greeks did. But if we step back for a moment and think about this, we would have to admit that the universe existing forever is an impossibility, both rationally and scientifically.
The Rational Perspective
The Universe could not have always existed because of how time works. Time moves in one direction. We all know this as a fact of life. Any suggestions that we can manipulate time is science fiction. You can go back and forth in the cardinal directions all you want, but you can never go back in time.
Anything that moves in only one direction, must have a beginning. Otherwise implies an infinity. An actual infinity—with a pre-eternal past and an eternal future—can never exist in the real world. Mathematical infinities are mere abstractions that don’t exist in the real world. The impossibility of pre-eternity precludes, logically, the existence of a beginning, since the two possibilities are naqids to one another: one must exist, but both cannot co-exist.
So this is the rational basis; and it suffices in itself. Recall that in the past, mechanisms of observation (science) were quite weak unlike today, and intelligent people did not rely on their conclusions. They thus relied mainly on reason. But if something is true, it will be supported by both reason and observation.
The Scientific Perspective
Scientifically, time is a representation and experience of motion between objects. When the ancients used to consider the Universe static, then it made sense that they also thought time was pre-eternal. But once human beings excelled in measuring things, physicists came upon a game-changer: the Universe is in a constant state of expansion. This means that they were closer together in the past. So if you chart their rate of motion on a cartesian plane, it allows you to measure backwards and eventually leads you to a moment when all stars and planets were together in one tiny space mashed up together. This would mark the zero point on the Y axis, and it represents the birth of the physical Universe and all matter, which also marks the beginning of time (our X axis) since time is none other than a representation of movements between objects. No objects, no movement, no time.
The conclusion that the Universe has a beginning opens up a whole new set of questions like: what created the universe, why did the universe need to exist, what are the qualities of the thing that created the universe and so on. It’s the Minor Premise of the Kalam Cosmological Argument:
Everything that has a beginning must have a cause.
The Universe had a beginning.
Therefore, the Universe must have a case which itself is uncaused.