Scientist discover that intergalactic travel is possible according to Einstein. The special theory of relativity suggests that given an unlimited amount of fuel intergalactic travel is possible.
Although an unlimited amount of fuel is not available, theoretically, according to the laws of physics, accelerating at such a rate that 1G astronauts throughout the journey could visit other galaxies.
The drawback is that by the time they returned Earth would be gone or all life on Earth would be extinct. This is because as their speed accelerates according to them, time dilates according to the people on Earth. Thus 76 years would seem like thousands of years on Earth.
So as time travel is indeed possible, it just has some small drawbacks to Earth dwellers.