We really hope that human science advances dramatically in the centuries to come, because we've examined a number of doomsday scenarios facing human existence, and with our current knowledge we don't have answers for any of them. Here's another one to add to the pile - our galaxy is eventually going to crash into its neighbor.
Andromeda is our closest celestial friend, and it seems to be in a hurry to meet us, because it's heading fight for us. The Milky Way must be just as keen though because that's heading for Andromeda just as fast. In fact, the two galaxies are driving dead on towards a collision at 250,000 miles per hour, with stars flying through the space between others and playing havoc with gravity, as well as creating a spectacular light show for anyone still living on Earth at the time. It's unknown whether it would be possible for life to survive the impact, and the galaxies would go on merging for approximately a billion years before settling down as a new, elliptical galaxy, but it will be around four billion years before we have to worry about if it they carry on at their current speeds.