The prevailing idea from the time of Plato and Aristotle up through 19 century was that the universe was eternal; it had no beginning and it has always been here. This view was challenged by at least two different discoveries of science.
The first challenge came from the discovery of the Second Law of Thermodynamics. This is the law of entropy which says that with no outside influence energy will dissipate over time and be used up. Heat up a pot of chili on the stove and then turn off the burner. After a while, the heat energy will be used up and the chili will cool to room temperature.
Time Magazine ran an article a few years ago: “How the Universe Will End.” It basically said that a day will come when all pockets of heat will go cold and all sources of light will burn up and the universe will grind to a halt and be motionless. Lights out, party is over.
You ask, so what does all of this prove in relationship to the eternal nature of the universe? Well, the point is if the universe will one day use up all its energy, that means that it cannot be eternal. Eternal means always has been always will be. If the universe will some day "die" then it's not eternal and therefore has a beginning.
The second scientific discovery that led cosmologists to believe the universe had a beginning started to surface in 1915 when Einstein’s came up with the General Theory of Relativity which basically states that the universe exists in a continual state of expansion or contraction. This theory was proven to be true in 1929 with empirical data by astronomer Edwin Hubble (yeah, the guy they named the telescope in Earth's orbit after).
Hubble discovered that galaxies were moving away from us at a speed that was proportional to their distance from the earth. Since this was true it meant that at some point in the past the entire universe began. As you trace this expansion back in time the universe grows denser and denser until finally the entire known universe is contracted down to a state of infinite density which, essentially, marks the beginning of the universe. Cosmologists call this The Singularity where all matter and energy, physical space and time came into being. This literally represents the origin of the universe from nothing. Hubble’s discovery was that the universe had an absolute beginning at some point in the finite past.
So, science has confirmed that the universe, time, space and matter exploded into existence: BANG!
Prior to 1929, faith and science were opposed on the need for a Creator to have given us our origins. If the universe is eternal then there was no need for a Creator to explain things. But after science proved that the universe had a definite beginning where time, space, and matter came into existence faith and science took one GIANT step toward each other. Science proved that the universe had a beginning and was created out of nothing.
Humm. Kind of reminds me of a verse from the Bible where it says, "In the beginning, God created the heavens and the earth. And God said, 'Let there be....'" (Genesis 1:1-2).
Robert Jastrow, director of NASA’s Goddard Institute for Space Studies once commented,
What is the ultimate solution to the origin of the universe? The answers provided by the astronomers are disconcerting and remarkable. Most remarkable of all is the fact that in science, as in the Bible, the world begins with an act of creation. For the scientist who has lived by his faith in the power of reason, the story ends like a bad dream. He has scaled the mountains of ignorance; he is about to conquer the highest peak; as he pulls himself over the final rock, he is greeted by a band of theologians who have been sitting there for centuries.The Big Bang theory, rather than making the notion of a Creator obsolete has served to demonstrate that it’s scientifically and philosophically more intelligent to believe that God created the universe a finite time ago … just as the Bible has always taught.
As it turns out, the big bang has a Banger who is bigger!