Dark matter has never been observed. Ever.
Mass and gravity doesn’t allow for the existence of galaxies, the math said galaxies should tear apart as they rotate, so science added a bunch of invisible / never observed mass to balance their equations. With this extra invisible mass, the math now says galaxies can exist.
That’s what dark matter is. That’s why it was hypothesised.
Dark matter is probably a bust theory, because the scientific method explicitly says not to add such (m+m.delta) fudge factors to your calcs, just to get the desired result. Especially if it requires adding something that has never been observed, ever. But dark matter hypothesis allows the rest of science to continue pretending gravity is real.
I know you asked about time, but I don’t think time is real, not how we understand it. Physics has the spacetime continuum now and there’s a nuance there that time is different in every location in space.
What that means is that time is more like temperate than a universal thing. Every point in space has its own time that is independent.
The older the universe gets the more scrambled up time becomes. Eg the core of planet Earth is today 2.5million years closer to the big bang than the Earth’s crust. This is also what entropy is (things becoming more disorderly), entropy is a law of thermodynamics and so its more robust a phenomenon than time. So it makes sense to try and think of time as behaving like entropy.
There is no universal time to travel back through, and entropy is irreversible.