Here is a brief introduction to what modern science has discovered about how the Universe came to be.
Behind everything, there is nothing. The most basic level of existence, underlying even the smallest particles, is a vacuum. Nothingness. The Void.
Pretty bleak sounding, but amazingly, quantum mechanics tells us that due to the infamous uncertainty principle, 'energy bubbles' can randomly appear out of nothing at all. It seems that on a quantum level, nothingness actually contains the potential for everything. However, the first law of thermodynamics (energy conservation) prohibits the creation or destruction of energy, so practically all of these bubbles take form as particle-antiparticle pairs, which annihilate each other almost instantly. It seems that the vacuum of empty space is actually roiling with such 'virtual' particles. Still, in a few rare 'quantum bubbles,' the positive energy of the electronuclear force is precisely equal to the negative energy of gravity, which means that the two aspects cancel each other out and the net energy of the 'bubble' is precisely zero. This type of 'baby universe' does not violate the first law of thermodynamics (energy conservation), and may therefore exist indefinitely. Amazing as it may seem, such phenomena beat the odds and literally result in something from nothing.
'Bubbles' which stick around longer than the tiniest fraction of a microsecond have the potential to undergo a process called 'inflation.' Over the course of an unimaginably short time, a quantum 'bubble' smaller than a single proton can grow to the size of a grapefruit or even larger. This expansion occurs geometrically (faster than the speed of light), and yet general relativity is not violated, because spacetime itself is expanding. This is the birth of a new Universe. The objects within the newborn Universe (as yet nothing but quantum fluctuations) remain where they were, they are simply stretched to an immense extent by the inflationary expansion. Some scientists believe that a second brief stage of inflation may then occur.
When certain physical conditions are met, inflation ceases and the new Universe bursts onto the scene, in a mammoth blast called the 'Big Bang.' The speed of expansion still approaches that of light closest to the wall of the now-tremendous 'bubble,' but the familiar laws of physics now take over. Symmetry of the 3 main forces other than gravity breaks and energy congeals into matter. The tiny fluctuations (inhomogenities) in the original 'bubble' have been stretched to fill the entire Universe, creating some areas where matter is distributed just slightly more densely than others. From these humble beginnings, the next few billion years are passed as gravity does its work of drawing matter together, eventually forming stars, galaxies, clusters of galaxies, and even clusters of clusters.
The rest, as they say, is history. Stars reach the end of their lives and explode, scattering the heavier elements produced as by-products of their nuclear fusion. This stardust eventually permeates the galaxies, and when new stars form, it surrounds them and forms planets. On at least one of those planets (and very likely others), those elements lead to the appearance of more and more complex life forms. This process is called self-organization - the inherent tendency of matter to arrange itself in more complex forms. Order emerges from chaos simply because it is only those areas which are capable of self-organizing (quantum bubbles which have zero net energy) which undergo inflation and become universes.
All of this happens on its own, for no particular reason, with no supernatural assistance. By studying the laws of Nature, scientists have been able to show how creation can occur in perfectly natural ways.
One more thing to think about - it is likely that the same conditions which caused the formation of our universe will occur again somewhere within it - in the vacuum of space, a new 'bubble' suitable for inflation will form simply by random chance, geometric expansion will occur, and a new 'Big Bang' will follow. Like an ever-growing fractal, it is possible that Universes spawn new Universes, in a cycle without beginning or end.
Can any Bible story top that? Compare this with the idea of creation in 6 days by a supernatural god, and decide for yourself which version you find more compelling.