Why are believers so certain there was a time when there was only void before the universe exists?

Is the universe eternal? Absolutely not. We know this is true because of the universally recognized Second Law of Thermodynamics (the law of energy decay or entropy). This law states that everything goes downhill from order to disorder, more usable energy to less. This law is the reason why heat flows from hot to cold, why you can’t have a perpetual motion machine, and why your vehicle will fall apart if it’s not kept up with. If someone doesn’t believe in the Second Law of Thermodynamics, just challenge them to live forever; even with this awesome machinery we have in our bodies, you will eventually wear out and die.

Astrophysicist Dr. Robert Jastrow (who was an agnostic) said, “Now three lines of evidence—the motions of the galaxies, the laws of thermodynamics, the life story of the stars—pointed to one conclusion; all indicated that the Universe had a beginning” (God and the Astronomers). And everything that has a beginning has a cause. The building you are in had a beginning, your computer had a beginning, you had a beginning, therefore there must have been a preceding and adequate cause.

We can see that the universe is running down and wearing out; the stars are burning up, the radioactive atoms are decaying, and so forth. As Psalm 102:26 says, the heavens “will wear out like a garment.” Eternal things obviously do not wear out because they would have had an infinite amount of time to come to their end. Since you cannot have an end without a beginning, the universe must have had a beginning. The astronomers know this and so (looking at the expanding universe) the majority of them, at the moment, believe in the “big bang” theory from that “singularity” or “cosmic egg” (the universe exploded into existence, if you will—or expanded). But, there is still a major problem—you have to explain where that “cosmic egg” came from. As it has been said, “There must be a cosmic chicken.”

In January 2012, the magazine "New Scientist" had an article called Death of the eternal cosmos: from the cosmic egg to the infinite multiverse, every model of the universe has a beginning. The article was talking about the work of cosmologist Dr. Alexander Vilenkin (an agnostic) whose team showed that every theory of the universe demands a beginning. The simple fact is, the universe is not eternal.

that's all we know, but when this test of the flesh is over soon, you can ask Him yourself......

Tes blagues, emmène-les chez les singes du Cirque Maximus !

God the father or creator has always been here and so has heaven or where spirit persons live. We cannot fully understand how they live or what they are actually like. It is like and ant thinking what is a human? What is his world like? The big bang is not true. Because they have proven some planets and even galaxies are older than others. So they did not all happen at the same time. But we can be sure that at some point our creator decided to make a planet habitable. The whole universe it says he holds in the palm of his hand. So he must be very big. We are probably in his back garden room as a display of stars and even earths moving around in his order. Just as and ant or flea would not understand if you told him I will take you to Africa away from this sand pile? So how would his life be different? We are a new experiment in the Universe and the humans with flesh and blood bodies is a new creation. How do we know that? Because the spirit persons came down here to see about us. If there was other planets he had made habitable they would not be so interested in us. Not to say eventually he will make other planets habitable. But for now he has to prove this experiment was not a failure.

By reciting the rosary every day with care and sincerity, a person can know numerous truths about God, creation and evolution.

I don't know what you mean by 'a void' before the universe because God has always existed and an angelic host existed before God made a material universe out of what cannot be seen. The infinitesimal densely compacted bit of 'stuff' that the BB theory speaks of could never have been seen by human eyes. Scientists speak of "a nugget that weighed a mere twenty pounds (9.07 kg) and was just 10 to the power of 26 cm in diameter (about one billionth of one billionth of one billionth of a centimetre)." I'm a believer who takes that to accord amazingly well with Hebrews 11:3. But for as long as God and angels existed, immaterial though they may be, existing in another dimension (or 2 or 3 or 23 dimensions for all we know), there is no 'void'. 

But Genesis 1:2 has been translated as the Earth "was without form and void" (or as "formless and empty"). The Hebrew reads "tohu… bohu" so Hebrew scholars are the people to ask about that. Clearly, matter abounded, but the verses depict darkness, formlessness, and a kind of emptiness with regard to planet Earth. So, God began by dispelling the darkness with light, organizing atmosphere, shaping rocks and seas, and starting to create life.  God formed light, water, sky, sea and land out of that which was 'formless'. He organized a heaven, water and sky and earth out of the cosmos, and he filled all of that - light out of darkness, order out of chaos, and filling that which appeared 'empty'. That's what the Genesis account shows and believers go by that ancient record.