It doesn't. Scientifically speaking,there is prove that shows that the Universe expands continuously, and it is doing so as we speak. What could bring an end to the Universe, as we know it, is that the opposite occurs: That we enter a state of entropy in which we stop expanding, and come inwards, basically crushing ourselves in. For entropy to occur, there must be a massive disruption in the thermodynamic balance of energy, preventing any physical reaction to take place. However, the more one reads about it, the more distant it seems that this would ever take place.
Yet, to be concise with your answer, the only way something as vast, unknown, and MAJOR as the Universe would ever end is by doing the opposite of what it normally does to maintain its balance. If the universe continuously expands, then the opposite of it would be what brings an end to it: To shrink.