Several theories have been put forward to explain the origin of the universe, none of which can be said to be sure for the moment. One of the most convincing and plausible theories is the big bang theory. Earlier it was believed that the universe has been in existence since infinite time and there is no beginning or end to it. But the big bang theory states that the universe did begin at a particular moment of time. This time frame is put by different experts in the region of about 12-15 billion years. Nobody knows what existed prior to that although several speculations do exist regarding the same.
Then suddenly a point in space which contained infinitely dense mass appeared and it started to expand at an enormous rate. Although this might look somewhat like a fairy tale story and many people might tend to disbelieve it, yet that is the best answer that scientists have been able to find till date.