Why is the universe so big?
The first verse of the Bible:
In the beginning God created the heaven and the earth
describes, in my opinion, the event that has been termed the "big bang". At that point, the universe began expanding.
This expansion has been continuing since moments after the bang, and is still continuing. The time period involved is extremely long from man's perspective - roughly 13.7 billion years. For a detailed treatment of the subject, read the Wikipedia article on The Chronology of the Universe.