From the unmanifested all the manifested proceed at the coming of the "day" ; at the coming of "night" they dissolve verily in that alone which is called the unmanifest. (Bhagavad Gita Chapter 8 verse 18)
In the worldly sense of the term, ‘creation’ is generally understood as the production of something new. Philosophically viewed creation has a subtler significance. A pot-maker creates pots out of mud. The act of ‘creation’ is only the production of a name and form, with some specific qualities, out of a raw material in which the name, form and qualities are already existing in an unmanifest condition. Hence, in Vedanta creation is nothing but a crystallization of the unmanifest dormant names, forms and qualities into their manifest forms of existence.
(Source: Holy Gita Ready Reference – Chinmaya International Foundation)