The big bang theory of cosmology says that the universe came into existence in a single event some ten or twenty billion years ago from a single, infinitely dense and hot pointlike ball of light, smaller than the smallest atom. In a fraction of a second, it expanded trillions of times creating all of the space, matter and energy that now makes up the galaxies and stars. The universe has been expanding and cooling since that initial event.
The Big Bang is often thought as the start of everything, including time, making any questions about what happened during it or beforehand nonsensical. Recently scientists have instead suggested the Big Bang might have just been the explosive beginning of the current era of the universe, hinting at a mysterious past.
One of the biggest mysteries in cosmology could be explained by a controversial theory in which the universe explodes into existence not just once, but repeatedly in endless cycles of death and rebirth.
Called the cyclic universe theory, it could potentially explain why a mysterious repulsive form of energy known as the "cosmological constant" and which is accelerating the expansion of the universe is several orders of magnitude smaller than predicted by the standard Big Bang model.
The constant was once much larger, but that its value decayed with each incarnation of the universe.
Because there can be endless cycles, the universe would be far older than the 14.7 billion years that scientists currently estimate.
A new theory for the origin of the Universe is intriguing astronomers with the idea that a "Big Splat" preceded the Big Bang.
It proposes that there may be an unseen parallel universe to ours.
The idea, which is still at the development stage, may provide hints about what happened before our Universe exploded into existence some 15 billion years ago.
M-theory looks at events before the Big Bang, proposing that the Universe has 11 dimensions, six of them rolled up into microscopic filaments that can, for all intents, be ignored.
M-theory brings the idea of more than one universe embedded in higher-dimensional space.
The action of the Universe takes place in five-dimensional space. Before the Big Bang occurred the Universe consisted of two perfectly flat four-dimensional surfaces.
One of these sheets is our Universe; the other, a "hidden" parallel universe.
The question is : Will we ever find out the truth?