Here is an excerpt from the upcoming biography of Srila Prabhupada, wherein the nature of the material world is described in brief and those who do not believe in such a spiritual world are informed of the foolishness of their limited idea.
The principle of creation is that the Supreme Lord, the Personality of Godhead, is by nature joyful, and He becomes many in order to enhance His transcendental joy. The living entities like ourselves, being part and parcel of the Supreme Lord, are meant to satisfy the senses of the Lord. Thus, whenever there is a discrepancy in that harmony, the living entity is immediately entrapped by maya (illusion). The external energy of the Lord is called the material world, and the kingdom of the internal energy of the Lord is called Vaikuntha, or the kingdom of God.
To a gross materialist, this kingdom of God, Vaikuntha, is certainly a mystery. But to an ignorant man everything is a mystery for want of sufficient knowledge. The kingdom of God is not a myth. Even the material planets, which float over our heads in the millions and billions, are still a mystery to the ignorant. Material scientists are now attempting to penetrate this mystery, and a day may come when the people of this earth will be able to travel in outer space and see the variegatedness of these millions of planets with their own eyes. In every planet there is as much material variegatedness as we find in our own planet.