The scientific answer to your question is would be that before the "big bang" all matter in the universe was condensed into a tiny fragment called "the singularity". Scientists and physicists are still trying to figure out what happened but somehow the matter exploded and began expanding. According to scientists, this occurred millions of years ago. But the expansion is still occurring so the matter in the universe has had quite a long time to grow larger. What is also interesting is that much of the matter in the universe is "dark", in other words invisible to our eyes. This means that we truly do not know how vast or immense the universe is, or even if other dimensions exist which contain other universes. You have an excellent question, one that mankind has been asking for centuries. We think we know part of the answer, but the entire answer remains unknown.