What exactly is a billion years? It's a hard concept to even imagine. To explain how long humans have been around in comparison to the universe, Carl Sagan used the metaphor of the calendar. If the universe began on January 1, the Milky Way galaxy wasn't formed until March, the sun and planets in our solar system didn't form until August, and single-celled life didn't appear until September. When did humans appear? Oh, around 10:48 p.m. on December 31. In other words, we've only been around for a little over an hour, and Columbus's voyage to the Americas happened a second ago.
We really have no concept of eternity. But let's assume the universe had a beginning. Scientists now believe the universe is about 14 billion years old. And from all evidence, the universe seems to be expending ever outward at an incredible rate. Can it expand forever without consequence? Some scientists suggest that it cannot. Think about filling up a balloon. Eventually it pops, and some believe this is the fate of the universe. This theory, known as "The Big Rip," speculates that the universe will eventually rip apart completely in 500 billion years, give or take.
Of course this hardly matters for Earth, as our own Sun will go Red Giant in 5 billion years (or April of Year 2 if you prefer Sagan's calendar).
Why does any of this matter? Maybe it doesn't, but think of all the people that think they will live forever. What concept of forever do they really have? And why should we believe that the universe will last for eternity?
500 Billion years is a long time, but it's not eternity.
If you believe in life after death, where do you see yourself in a billion years? If that concept is too insane to imagine, where do you see yourself in a thousand years?
If you don't believe in life after death, why not? How do you explain people that do?
"Eternity's a terrible thought. I mean, where's it all going to end?" -- Tom Stoppard