How the Universe was born?

In Big Bang
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About 13.7 billion years ago all the materia of the universe was condensed in one spot, called singularity, which size was blinding small and density infinite. When singularity was condensed to as small it can be, it exploded. It's called the Big Bang, because it was the biggest explosion of the universe. There was no sound in it, because there were no air in which the sonic waves would travel, so the big "bang" is relatively misleading term. In just a fraction of second the whole universe spread out of that spot. It wasn't like we know it now, just large amount of chemical elements. Elements mixed together creating new elements and forming objects. Elements were first in gas phase, but while cooling, some of them changed to liquid and solid form. Objects was first like fogs, then they condensed to solid objects. Gravity shaped the objects to spherical shape, like planets, planetoids (small planet-like objects), moons etc. If the object was heavy enough in physical mass, a nuclear reaction started in its core. The object started to radiate and it has become a star.
Stars born in nebulas (fogs full of materials for stars), when the star is condensed, there usually is lots of remain material (gas) around it. Gravity shapes the gas in the form of a disc that orbits around the star. The materia in the disc accumulates and form planets. If the spherical objects were near some object with greater mass, it probably started to orbit that object, so it became a moon. At first planets were gas, then liquid, then they cooled to solid objects. Planets heavy enough stayed in gas form. Planets have several layers inside them. For example Earths inner core is solid, outer core liquid, mantle viscous solid, outer mantle solid and crust solid. Most of the planets have just one core. Stars have layers also, core, mantle and surface.
Stars accumulated in galaxies, large drift of stars, and planets formed solar systems around the stars. This creative process is always under way, new stars and planets are constantly forming while others are being destroyed. The natural cause of universe.

No one can know what has happened before the Big Bang, because the time itself began then. Humans can make sharper and sharper telescopes and look further and further back in time, but we can't look back at the time, when time hasn't begun.