Intergalactic travel
Intergalactic travel is travel between galaxies, and it is even more difficult than interstellar travel. At the speed of light, travelling from Earth in the Milky Way galaxy to the Andromeda Galaxy (the nearest major galaxy) would take roughly three million years from the perspective of observers on Earth, but would take an arbitrarily short amount of time for the traveller, depending on exactly how close to the speed of light his vessel is traveling.