The current understanding by scientists and astronomers is that our physical Universe did not exist until some 13.7 billion years ago, when somehow an infinitely dense, infinitely small point called a singularity suddenly materialized and exploded into the present universe. They have ended up calling this the 'Big Bang' Theory.
What is interesting to me, is that if astronomers are correct about this theory, then not only did the Universe in fact have a beginning, but even more importantly, it had to have been formed in a highly ordered state according to it's own physical laws.
One dictionary describes cosmological entropy as the "tendency for the Universe to attain a state of maximum homogeneity in which all matter is at a uniform temperature". In other words, over time, forces within the Universe itself act to rearrange things, things will get more messy or disordered than ordered. This also means that there is a difference between the past and the future: the past was more ordered and the future will be less ordered because this is the most likely way for things to play out.
If you take a university physics degree this is what you will learn about entropy and the arrow of time - the past had a lower entropy than the future; ordered things become disordered as time ticks by. So one might legitimately ask where all the order in the Universe came from in the first place.
Some scientists admit to believing in God because of their observations of what appears to be a fine-tuning of our Universe in general. It is equipped with fixed physical laws and with natural constants that are precisely and ideally suited to support a planet like ours and all the life on it. So how did the Earth get here? It's very ordered. And how did the Milky Way appear if it is composed of billions of ordered worlds orbiting around billions of ordered stars? There must have been some reason why the Universe began in such a highly ordered state, such that it can gradually fall into place this way. The Universe began with sufficient order in the bank to allow planets, stars and galaxies to appear. And the precise settings of the four fundamental physical forces (electromagnetism, gravity, strong nuclear force, and weak nuclear force), affect every object in the universe. They are set and balanced so precisely that even slight changes could render the universe lifeless. So there must have been a LOT of order to begin with.
In other words, the Universe was born in a highly ordered state, and there should be a reason for that. It is unlikely to have been chance. Since the Universe is far less ordered today than it was 13.7 billion years ago, this means that it would have been far more likely that our Universe popped into existence a billionth of a second ago, fully formed with planets, stars, galaxies and people, than it was that the Universe popped into existence at the Big Bang in a highly ordered state. Yet, that exactly is what astronomers say happened.
In summary, SOMETHING had to have always existed in order for the physical Universe to be here at all. Because the design of the Universe is SO ordered and had to have begun in such a precise fashion, this lends significant evidence of a Designer. As the Bible stated the obvious millenia ago:
"Of course, every house is constructed by someone, but he that constructed all things is God." (Heb. 3:4; also see Romans 1:20; Isaiah 40:26 and Psalm 19:1)
Undoubtedly, the concept of Someone never having a beginning is completely baffling and foreign to our human minds. But we are forced to perceive things through only our physical Universe. God, on the other hand is described as a "Spirit" (John 4:24) and His thoughts are higher than ours (Isa. 55:9). Perhaps someday we will come to understand. But until then, we can be satisfied to know for all intents and purposes that God truly has existed "from time indefinite to time indefinite". (Ps. 90:2)
For more, see:
The "coincidences" of extra protons, and the very small mass difference between a neutron and proton, etc. (Search For Bible Truths)
The "Just Right" Status of the Gravitational Force (Search For Bible Truths)
Why Do Some Scientists Believe in God? (Search For Bible Truths)
What is the Big Bang Theory? (Search For Bible Truths)
How Did the Universe and Life Originate? (g02 6/8 pp. 4-7; Watchtower Online Library)
Is There Really a Most High Creator? (pc pp. 9-12; Watchtower Online Library)