The current consensus among scientists is that there was a "Big Bang" at the very beginning of the universe, an initial explosion that generated time, space, and all the matter and energy that exists. In the immediate aftermath of this event, fundamental forces such as gravity emerged, and then fundamental particles. Ever since this time, believed to be at least 15 billion years ago, the universe has been expanding -- more slowly at times, faster at others. Scientific observations suggest that this expansion might ultimately either drive our Milky Way galaxy to merge with a neighboring galaxy, Andromeda, or cause the two to rip each other apart. Although this is bad news for our galaxy, expansion won't stop there. The final timeline entry describes the expected scene trillions of trillions of years in the future, when universal energy will finally have been expended, and the universe will end.