Tough question this, probably better answered by a Physicist. I’ll give it a go though.
According to the big bang, our universe is spreading out all over the place, implying it started at one point somewhere a really long time ago (~13.7 billion years). So, all of the matter in the universe existed in this single point. This would be really cramped for everyone. Also, the mass of this single point was so immense that it meant time could not progress either (Einstein’s Theory of Relativity).