There are some slight problems with the Big Bang theory, which states that all matter in the universe exploded from a small point about twenty billion years ago.
For example, there is the Second Law of Thermodynamics, which states that unless energy is introduced into a particular state, the energy of that state will always be less than the initial state. This is known as entropy.
Thus, in order to explode, any original matter (wherever it came from) would have had to create energy in some way. To form planets, suns and entire galaxies, which is entropy running backwards, it would in effect need to create vast amounts of energy from nowhere, as well as vast amounts of matter (which is also impossible).
Where did that original cosmic egg come from? Where did its energy come from? Why did it suddenly explode? Where did it get sufficient matter for hundreds of thousands of galaxies from?
Also note: if the universe originated from a big bang, matter should be evenly spread out. It isn't. We note large densities of matter in some places, and virtually empty spaces between galaxies in others.