Time dilation (the phenomenon you describe) is not the same as time travel. Time travel consists of reversing the movement of a system from higher energy to greater entropy. It would essentially allow you to unring a bell.
Dark matter is a ridiculous theory concocted to explain the gravitational discrepancy you noted. It’s a perfect example of forcing reality to fit the equation, since it is “observable” only by its absence.
Yep. Dark matter and dark energy are both fudge factors intended to bridge the gap between observation and theory.
Ultimately a better theory is needed, but it will be (is being?) bitterly opposed because of the threat it poses to orthodoxy. Even observation is often persecuted if it fails to support the orthodox theories.