The answer is:
Time and Space Travel exists.
Time Travel exists because we travel through time every moment of our lives, and we travel through space every time we move around.
Therefore, if on a small scale it is possible, we can assume that it is possible on a large scale. However, by the time we get to a stage where time travel is possible we will only go forward, never backward, purely because we are still discovering things, if backwards time travel was possible we would already know about it.