Take a close look at that pattern. You will find that it is made up of only two shapes. They are then duplicated and enlarged a few times until the two shapes look like they had merged with each other.
That was the first layer, the second layer consist of a radial gradient.
And well I believe that is about it.
Good luck... Oh BTW if you plan on doing this for anything other than a learning experience, change the shapes to your own. Making an exact copy is a copyright infringement.