The Universe is expanding, this is a known fact. According to The Big Bang Theory (not the series, the actual scientific theory), the Universe originated in an unimaginably massive Big Bang, and material got thrown outwards from a single point.
It is thus easy to imagine that the Universe has been expanding for the last 13.8 billion years. A blast of any kind throws material outwards so expansion is quite logical.
Interesting thing is that instead of slowing down, the expansion of the universe has accelerated. The reason for this is Dark Energy. This form of mysterious energy has a repulsive effect greater than gravity and thus makes the entire Universe expand.
But scientists now think that the Universe will start contracting remarkably soon. They have put forth their research in the journal Scientific Proceedings of National Academy of Sciences.
The team of scientists behind this study has built a model of the Universe in which Dark Energy is not a constant force of nature but an entity called quintessence. This entity can decay over time.
So the scientists say that the acceleration of the Universe would stop in next 65 million years. In next 100 million years (a very short time when space is considered) it will stop expanding and start contracting. And billions of years after that, the Universe may meet its death (or a rebirth of time and space).